title: Replacing masked values (--) with a Null or None value using fiil_value from ma numpy in Python
body: <p>Is there a way to replace a masked value in a numpy masked array as a null or None value? This is what I have tried but does not work.</p>
  
  <pre><code>for stars in range(length_masterlist_final):
      ....
      star = customSimbad.query_object(star_names[stars])
          #obtain stellar info.
          photometry_dataframe.iloc[stars,0] = star_IDs[stars]
          photometry_dataframe.iloc[stars,1] = star_names[stars]
  
          photometry_dataframe.iloc[stars,2] = star['FLUX_U'][0]
          #Replace "--" masked values with a Null (i.e., '') value.
          photometry_dataframe.iloc[stars,2] = ma.filled(photometry_dataframe.iloc[stars,2], fill_value=None)
  .....
  photometry_dataframe.to_csv(output_dir + "simbad_photometry.csv", index=False, header=True, na_rep='NaN')
  </code></pre>
  
  <p>specifically </p>
  
  <pre><code>(photometry_dataframe.iloc[stars,2] = ma.filled(photometry_dataframe.iloc[stars,2], fill_value=None)) 
  </code></pre>
  
  <p>produces </p>
  
  <pre><code>'MaskedConstant' object has no attribute '_fill_value'
  </code></pre>
  
  <p>I want to replace masked values '--' with '' when I output the dataframe as a csv file. One work around is to read the outputted csv file back into python and replace '--' with '', but this is a horrible solution. There must be a better solution. I don't want masked values printed as '--' in the csv file.</p>