Change np.isnan to pd.isnull in conventions.py to handle _FillValue = ''

At a number of spots in conventions.py, np.isnan was being used
to check possible fill values.  However, np.isnan is not implemented
for string types.  So, I've replaced np.isnan with pd.isnull.

Alas, I haven't been able to update the tests to cover _FillValue = '',
because doing so would involve significant changes throughout the
code base.