title: Drop masked rows from Dask dataframe when saving to file
body: <p>I have several large csv files that I'm processing with <a href="https://dask.org/" rel="nofollow noreferrer">Dask</a>. I need to mask these files according to some condition, and save the masked array <strong>without</strong> the masked rows. What I currently do is:</p>
  <pre><code>from dask import dataframe as dd
  df = dd.read_csv(filename)
  msk = ... # some condition
  df = df.mask(msk).compute()
  df.to_csv(&quot;{}_sample.csv&quot;.format(filename), index=False)
  </code></pre>
  <p>The masking works, but the resulting file still contains the masked rows as empty rows, i.e:</p>
  <pre><code>...
  18.702003,0.005,79.428,9.999001250124936,0.5203728231202968,0.2673634806190893,-0.58664254749603
  19.102915,0.069,77.81,9.999238070973211,-0.6233755821087494,0.3886258651317274,-3.88229321744741
  ,,,,,,,,,,,,
  ,,,,,,,,,,,,
  ,,,,,,,,,,,,
  ,,,,,,,,,,,,
  20.388945,0.08199999999999999,77.50999999999998,9.999336227970336,0.35936464745549523,1.23090232
  ,,,,,,,,,,,,
  ...
  </code></pre>
  <p>I've looked at the <a href="https://docs.dask.org/en/latest/dataframe-api.html#dask.dataframe.to_csv" rel="nofollow noreferrer"><code>to_csv</code></a> function but I see no option to drop these empty/masked rows.</p>