Caught case where the pcolormesh has masked pcolor values.

There is a bug in matplotlib which restricts pcolor plots to data arrays which have at least one non-masked value (). This is a problem for cartopy when it tries to handle a warped pcolormesh, as inbetween the wrapping, it draws a pcolor. This change adds a check to ensure that a pcolor being drawn actually contains non-masked data, otherwise it just skips the step.


```
import cartopy.crs as ccrs
import matplotlib.pyplot as plt
import numpy as np

import numpy.random

data = np.ma.masked_all((30, 40))

plt.axes(projection=ccrs.PlateCarree())
plt.pcolormesh(np.linspace(-90, 90, 40), np.linspace(0, 360, 30), data)
plt.show()
```