Spurious(?) all-NaN slice RuntimeWarning #35
Description
For example, the first example given in the README (using the 'scatter_density' method) currently emits
.../lib/python3.9/site-packages/mpl_scatter_density/generic_density_artist.py:77: RuntimeWarning: All-NaN slice encountered
vmin = self._density_vmin(array)
.../lib/python3.9/site-packages/mpl_scatter_density/generic_density_artist.py:82: RuntimeWarning: All-NaN slice encountered
vmax = self._density_vmax(array)
when run with numpy 1.20.1, matplotlib 3.3.0, mpl-scatter-density 0.7.