GeoRaster reduce doesn't support fully masked rasters

[drnextgis]

>>> import numpy as np
>>> from test_georaster import some_raster
>>> fully_masked = some_raster.copy_with(
...     image=np.ma.array(some_raster.image.data, mask=True))
>>> fully_masked.min()  # ValueError
>>> fully_masked.max()  # ValueError
>>> fully_masked.sum()  # [0], should be [masked]
>>> fully_masked.mean() # [nan], should be [masked]
>>> fully_masked.var()  # [nan], should be [masked]
>>> fully_masked.std()  # [masked] OK!