numpy.ma default fill value for strings not appropriate for S1 and S2

For string arrays of type S1 or S2, the default fill value in Numpy masked arrays (N/A) is not appropriate:

In [1]: from numpy import ma

In [2]: x = ma.array(['a', 'b', 'c'], dtype='S1', mask=[1,0,0])

In [3]: x.filled()
Out[3]: 
array(['N', 'b', 'c'], 
      dtype='|S1')

In [4]: x.get_fill_value()
Out[4]: 'N/A'

this causes issues for codes that assume the fill value is present in the filled array and indicated masked values. It's not clear to me what the fill value should be, but I'd just thought I'd raise the question.

cc @taldcroft

Start by reproducing the masked-array example with S1 and S2 string dtypes and trace how numpy.ma selects the default fill value. The issue does not identify a file or test; done requires an agreed fill value that remains appropriate for both widths and tests showing masked values are distinguishable after filling.