pygmt.sphdistance: data should be an optional parameter if voronoi is used

**Description of the problem**

pygmt.sphdistance will give a `GMTInvalidInput: No input data provided` error if `data`is not used. However, the purpose of the `voronoi` parameter is to allow precomputed Voronoi polygons to be provided instead of raw data. So, the `data` parameter should be made optional if `Voronoi` is set.

**Full code that generated the error**

precompute using gmt's sphtriangulate:

```bash
gmt sphtriangulate @gshhs_c.txt -Qv -D > tt.pol
```
```python
import pygmt
fig = pygmt.Figure()
grid = pygmt.sphdistance(voronoi="tt.pol", region="g", spacing=1, unit="k")
```

**Full error message**

```
67 """
68 if data is None and x is None and y is None:
---> 69 raise GMTInvalidInput("No input data provided.")
70 if data is not None and (x is not None or y is not None or z is not None):
71 raise GMTInvalidInput("Too much data. Use either data or x and y.")

GMTInvalidInput: No input data provided.
```

**System information**

Please paste the output of `python -c "import pygmt; pygmt.show_versions()"`:

```
pygmt: v0.5.0
gmt: 6.3.0
```