Cycle detected in layout in Groups visualization

I think that this is happening when the graph is empty

```python-traceback
[ 287.943434] cloud-init[978]: bokeh.util.tornado - ERROR - Error thrown from periodic callback:
[ 287.947840] cloud-init[978]: bokeh.util.tornado - ERROR - Traceback (most recent call last):
[ 287.950235] cloud-init[978]: File "/opt/conda/lib/python3.8/site-packages/tornado/gen.py", line 526, in callback
[ 287.951940] cloud-init[978]: result_list.append(f.result())
[ 287.952196] cloud-init[978]: File "/opt/conda/lib/python3.8/site-packages/bokeh/server/session.py", line 67, in _needs_document_lock_wrapper
[ 287.952648] cloud-init[978]: result = func(self, *args, **kwargs)
[ 287.952874] cloud-init[978]: File "/opt/conda/lib/python3.8/site-packages/bokeh/server/session.py", line 195, in with_document_locked
[ 287.953102] cloud-init[978]: return func(*args, **kwargs)
[ 287.953806] cloud-init[978]: File "/opt/conda/lib/python3.8/site-packages/bokeh/document/document.py", line 1164, in wrapper
[ 287.954100] cloud-init[978]: return doc._with_self_as_curdoc(invoke)
[ 287.954560] cloud-init[978]: File "/opt/conda/lib/python3.8/site-packages/bokeh/document/document.py", line 1150, in _with_self_as_curdoc
[ 287.954759] cloud-init[978]: return f()
[ 287.955011] cloud-init[978]: File "/opt/conda/lib/python3.8/site-packages/bokeh/document/document.py", line 1163, in invoke
[ 287.955233] cloud-init[978]: return f(*args, **kwargs)
[ 287.955472] cloud-init[978]: File "/opt/conda/lib/python3.8/site-packages/distributed/dashboard/components/__init__.py", line 78, in
[ 287.955775] cloud-init[978]: doc.add_periodic_callback(lambda: update(ref), interval)
[ 287.955975] cloud-init[978]: File "/opt/conda/lib/python3.8/site-packages/bokeh/core/property/validation.py", line 93, in func
[ 287.956194] cloud-init[978]: return input_function(*args, **kwargs)
[ 287.956458] cloud-init[978]: File "/opt/conda/lib/python3.8/site-packages/distributed/dashboard/components/__init__.py", line 86, in update
[ 287.956687] cloud-init[978]: comp.update()
[ 287.956941] cloud-init[978]: File "/opt/conda/lib/python3.8/site-packages/bokeh/core/property/validation.py", line 93, in func
[ 287.957183] cloud-init[978]: return input_function(*args, **kwargs)
[ 287.957419] cloud-init[978]: File "/opt/conda/lib/python3.8/site-packages/distributed/dashboard/components/scheduler.py", line 2038, in update
[ 287.957916] cloud-init[978]: self.nodes_layout, self.arrows_layout = self.update_layout()
[ 287.958170] cloud-init[978]: File "/opt/conda/lib/python3.8/site-packages/bokeh/core/property/validation.py", line 93, in func
[ 287.958607] cloud-init[978]: return input_function(*args, **kwargs)
[ 287.958833] cloud-init[978]: File "/opt/conda/lib/python3.8/site-packages/distributed/dashboard/components/scheduler.py", line 1973, in update_layout
[ 287.959293] cloud-init[978]: order = dask.order.order(
[ 287.959515] cloud-init[978]: File "/dask/order.py", line 120, in order
[ 287.959998] cloud-init[978]: raise RuntimeError(
[ 287.960259] cloud-init[978]: RuntimeError: Cycle detected between the following keys:
[ 287.960473] cloud-init[978]: ->
[ 288.007375] cloud-init[978]: distributed.utils - ERROR - Cycle detected between the following keys:
```

cc @ncclementi