step plot broken when using streaming dataframe

Hi,

I'd just like to report that it appears the step plot doesn't work with streaming dataframes.

#### ALL software version info
(this library, plus any other relevant software, e.g. bokeh, python, notebook, OS, browser, etc)
Latest versions of the main libraries (I also tried some older versions of hvplot because I believe this has worked in the past but couldn't find any combination that worked in my brief attempts)

```
Name: hvplot
Version: 0.6.0

Name: holoviews
Version: 1.13.3

Name: streamz
Version: 0.5.5

Name: bokeh
Version: 2.1.1
```

#### Description of expected behavior and the observed behavior
Step plot should work or hvplot should raise a more informative exception (step plot not allowed with streaming dataframes?)

#### Complete, minimal, self-contained example code that reproduces the issue

```
import pandas as pd
from streamz import Stream
import hvplot.streamz

s = Stream()
sdf = s.to_dataframe(example=pd.DataFrame(columns=['x', 'y']))
sdf.hvplot.step() # replace with any other plot type works fine
```

#### Stack traceback and/or browser JavaScript console output
```
---------------------------------------------------------------------------
IndexError Traceback (most recent call last)
./venv/lib/python3.8/site-packages/IPython/core/formatters.py in __call__(self, obj, include, exclude)
968
969 if method is not None:
--> 970 return method(include=include, exclude=exclude)
971 return None
972 else:

./venv/lib/python3.8/site-packages/holoviews/core/dimension.py in _repr_mimebundle_(self, include, exclude)
1310 combined and returned.
1311 """
-> 1312 return Store.render(self)
1313
1314

./venv/lib/python3.8/site-packages/holoviews/core/options.py in render(cls, obj)
1393 data, metadata = {}, {}
1394 for hook in hooks:
-> 1395 ret = hook(obj)
1396 if ret is None:
1397 continue

./venv/lib/python3.8/site-packages/holoviews/ipython/display_hooks.py in pprint_display(obj)
280 if not ip.display_formatter.formatters['text/plain'].pprint:
281 return None
--> 282 return display(obj, raw_output=True)
283
284

./venv/lib/python3.8/site-packages/holoviews/ipython/display_hooks.py in display(obj, raw_output, **kwargs)
256 elif isinstance(obj, (HoloMap, DynamicMap)):
257 with option_state(obj):
--> 258 output = map_display(obj)
259 elif isinstance(obj, Plot):
260 output = render(obj)

./venv/lib/python3.8/site-packages/holoviews/ipython/display_hooks.py in wrapped(element)
144 try:
145 max_frames = OutputSettings.options['max_frames']
--> 146 mimebundle = fn(element, max_frames=max_frames)
147 if mimebundle is None:
148 return {}, {}

./venv/lib/python3.8/site-packages/holoviews/ipython/display_hooks.py in map_display(vmap, max_frames)
204 return None
205
--> 206 return render(vmap)
207
208

./venv/lib/python3.8/site-packages/holoviews/ipython/display_hooks.py in render(obj, **kwargs)
66 renderer = renderer.instance(fig='png')
67
---> 68 return renderer.components(obj, **kwargs)
69
70

./venv/lib/python3.8/site-packages/holoviews/plotting/renderer.py in components(self, obj, fmt, comm, **kwargs)
386 doc = Document()
387 with config.set(embed=embed):
--> 388 model = plot.layout._render_model(doc, comm)
389 if embed:
390 return render_model(model, comm)

./venv/lib/python3.8/site-packages/panel/viewable.py in _render_model(self, doc, comm)
416 if comm is None:
417 comm = state._comm_manager.get_server_comm()
--> 418 model = self.get_root(doc, comm)
419
420 if config.embed:

./venv/lib/python3.8/site-packages/panel/viewable.py in get_root(self, doc, comm)
645 """
646 doc = doc or _curdoc()
--> 647 root = self._get_model(doc, comm=comm)
648 self._preprocess(root)
649 ref = root.ref['id']

./venv/lib/python3.8/site-packages/panel/layout.py in _get_model(self, doc, root, parent, comm)
118 if root is None:
119 root = model
--> 120 objects = self._get_objects(model, [], doc, root, comm)
121 props = dict(self._init_properties(), objects=objects)
122 model.update(**self._process_param_change(props))

./venv/lib/python3.8/site-packages/panel/layout.py in _get_objects(self, model, old_objects, doc, root, comm)
108 else:
109 try:
--> 110 child = pane._get_model(doc, root, model, comm)
111 except RerenderError:
112 return self._get_objects(model, current_objects[:i], doc, root, comm)

./venv/lib/python3.8/site-packages/panel/pane/holoviews.py in _get_model(self, doc, root, parent, comm)
225 plot = self.object
226 else:
--> 227 plot = self._render(doc, comm, root)
228
229 plot.pane = self

./venv/lib/python3.8/site-packages/panel/pane/holoviews.py in _render(self, doc, comm, root)
284 kwargs = {}
285
--> 286 return renderer.get_plot(self.object, **kwargs)
287
288 def _cleanup(self, root):

./venv/lib/python3.8/site-packages/holoviews/plotting/bokeh/renderer.py in get_plot(self_or_cls, obj, doc, renderer, **kwargs)
71 combining the bokeh model with another plot.
72 """
---> 73 plot = super(BokehRenderer, self_or_cls).get_plot(obj, doc, renderer, **kwargs)
74 if plot.document is None:
75 plot.document = Document() if self_or_cls.notebook_context else curdoc()

./venv/lib/python3.8/site-packages/holoviews/plotting/renderer.py in get_plot(self_or_cls, obj, doc, renderer, comm, **kwargs)
235 init_key = tuple(v if d is None else d for v, d in
236 zip(plot.keys[0], defaults))
--> 237 plot.update(init_key)
238 else:
239 plot = obj

./venv/lib/python3.8/site-packages/holoviews/plotting/plot.py in update(self, key)
898 def update(self, key):
899 if len(self) == 1 and ((key == 0) or (key == self.keys[0])) and not self.drawn:
--> 900 return self.initialize_plot()
901 item = self.__getitem__(key)
902 self.traverse(lambda x: setattr(x, '_updated', True))

./venv/lib/python3.8/site-packages/holoviews/plotting/bokeh/element.py in initialize_plot(self, ranges, plot, plots)
2221 if self.tabs:
2222 subplot.overlaid = False
-> 2223 child = subplot.initialize_plot(ranges, plot, plots)
2224 if isinstance(element, CompositeOverlay):
2225 # Ensure that all subplots are in the same state

./venv/lib/python3.8/site-packages/holoviews/plotting/bokeh/element.py in initialize_plot(self, ranges, plot, plots, source)
1337 self.handles['plot'] = plot
1338
-> 1339 self._init_glyphs(plot, element, ranges, source)
1340 if not self.overlaid:
1341 self._update_plot(key, plot, style_element)

./venv/lib/python3.8/site-packages/holoviews/plotting/bokeh/element.py in _init_glyphs(self, plot, element, ranges, source)
1281 else:
1282 style = self.style[self.cyclic_index]
-> 1283 data, mapping, style = self.get_data(element, ranges, style)
1284 current_id = element._plot_id
1285

./venv/lib/python3.8/site-packages/holoviews/plotting/bokeh/chart.py in get_data(self, element, ranges, style)
359
360 if 'steps' in self.interpolation:
--> 361 element = interpolate_curve(element, interpolation=self.interpolation)
362 data = {x: element.dimension_values(xidx),
363 y: element.dimension_values(yidx)}

./venv/lib/python3.8/site-packages/param/parameterized.py in __new__(class_, *args, **params)
2810 inst = class_.instance()
2811 inst.param._set_name(class_.__name__)
-> 2812 return inst.__call__(*args,**params)
2813
2814 def __call__(self,*args,**kw):

./venv/lib/python3.8/site-packages/holoviews/core/operation.py in __call__(self, element, **kwargs)
197 kwargs['per_element'] = self._per_element
198 kwargs['link_dataset'] = self._propagate_dataset
--> 199 return element.apply(self, **kwargs)
200
201

./venv/lib/python3.8/site-packages/holoviews/core/accessors.py in pipelined_call(*args, **kwargs)
43
44 try:
---> 45 result = __call__(*args, **kwargs)
46
47 if not in_method:

./venv/lib/python3.8/site-packages/holoviews/core/accessors.py in __call__(self, apply_function, streams, link_inputs, link_dataset, dynamic, per_element, **kwargs)
202 if hasattr(apply_function, 'dynamic'):
203 inner_kwargs['dynamic'] = False
--> 204 new_obj = apply_function(self._obj, **inner_kwargs)
205 if (link_dataset and isinstance(self._obj, Dataset) and
206 isinstance(new_obj, Dataset) and new_obj._dataset is None):

./venv/lib/python3.8/site-packages/holoviews/core/operation.py in __call__(self, element, **kwargs)
192 elif ((self._per_element and isinstance(element, Element)) or
193 (not self._per_element and isinstance(element, ViewableElement))):
--> 194 return self._apply(element)
195 elif 'streams' not in kwargs:
196 kwargs['streams'] = self.p.streams

./venv/lib/python3.8/site-packages/holoviews/core/operation.py in _apply(self, element, key)
130
131 element_pipeline = getattr(element, '_pipeline', None)
--> 132 ret = self._process(element, key)
133
134 for hook in self._postprocess_hooks:

./venv/lib/python3.8/site-packages/holoviews/operation/element.py in _process(self, element, key)
965
966 def _process(self, element, key=None):
--> 967 return element.map(self._process_layer, Element)
968
969

./venv/lib/python3.8/site-packages/holoviews/core/data/__init__.py in pipelined_fn(*args, **kwargs)
214
215 try:
--> 216 result = method_fn(*args, **kwargs)
217
218 op = method_op.instance(

./venv/lib/python3.8/site-packages/holoviews/core/data/__init__.py in map(self, *args, **kwargs)
1181
1182 def map(self, *args, **kwargs):
-> 1183 return super(Dataset, self).map(*args, **kwargs)
1184 map.__doc__ = LabelledData.map.__doc__
1185

./venv/lib/python3.8/site-packages/holoviews/core/dimension.py in map(self, map_fn, specs, clone)
703 return deep_mapped
704 else:
--> 705 return map_fn(self) if applies else self
706
707

./venv/lib/python3.8/site-packages/holoviews/operation/element.py in _process_layer(self, element, key)
959 x = x.astype(dt_type)
960 dvals = tuple(element.dimension_values(d) for d in element.dimensions()[1:])
--> 961 xs, dvals = INTERPOLATE_FUNCS[self.p.interpolation](x, dvals)
962 if is_datetime:
963 xs = xs.astype(dt_type)

./venv/lib/python3.8/site-packages/holoviews/operation/element.py in pts_to_midstep(cls, x, values)
921
922 steps[1:-1:2] = steps[2::2] = x[:-1] + (x[1:] - x[:-1])/2
--> 923 steps[0], steps[-1] = x[0], x[-1]
924
925 val_arrays = []

IndexError: index 0 is out of bounds for axis 0 with size 0
```

#### Screenshots or screencasts of the bug in action