title: Output for nested list comprehension as empty list instead of list with None
body: <p>I have a nested list which I am trying to parse into datetime objects.
  Some strings have invalid formats which should be an empty list in the output.</p>
  <pre><code>input = [
     ['20210804:1700', '20210805:1600'],
     ['20210805:1700', '20210806:1600'],
     ['20210807:CLOSED']]
  </code></pre>
  <p>So far I have the following.</p>
  <pre><code>def _helper(x):
      try:
          return dt.datetime.strptime(x, '%Y%m%d:%H%M')
      except Exception:
          return
  
  output = [[_helper(i) for i in group] for group in input]
  </code></pre>
  <p>Currently the output is as follows</p>
  <pre><code>[[datetime.datetime(2021, 8, 4, 17, 0), datetime.datetime(2021, 8, 5, 16, 0)],
  [datetime.datetime(2021, 8, 5, 17, 0), datetime.datetime(2021, 8, 6, 16, 0)], 
  [None]]
  </code></pre>
  <p>I would like the output to have an empty list <code>[]</code> instead of <code>[None]</code>.
  How can I do that?
  Thanks</p>