title: Django filter - is DateTimeField filled
body: <p>to my model I added a simply DateTimeField:</p>
  <pre><code>expired = models.DateTimeField(default=None)
  </code></pre>
  <p>. The value of the field can be either None or a Datetime.</p>
  <p>I'd like to filter for objects where the expired is filled with any datum, however I'm struggling to find the right filter.</p>
  <p>I think I tried all the combinations of filter / exclude and expired__isnull=True / expired=None, but I never get back the exact number.</p>
  <p>What's the right way to filter if the field has a DateTime in it, or not?</p>
  <p>Django: 1.11.16</p>
  <p>Thanks.</p>
  <hr />
  <p>In my model there're 2122 lines:</p>
  <pre><code>Counter(Obj.objects.filter().values_list('expired'))
  Counter({(datetime.datetime(2021, 9, 24, 1, 6, 50),): 1,
       (datetime.datetime(2021, 9, 24, 1, 6, 51),): 1,
       (datetime.datetime(2021, 9, 24, 1, 6, 32),): 1,
       (datetime.datetime(2021, 9, 24, 1, 12, 3),): 1,
       (datetime.datetime(2021, 9, 24, 1, 12, 44),): 1,
       (datetime.datetime(2021, 12, 4, 1, 31, 25),): 1,
       (datetime.datetime(2021, 12, 4, 1, 37, 49),): 1,
       (datetime.datetime(2021, 12, 4, 1, 9, 55),): 1,
       (None,): 2087,
       (datetime.datetime(2021, 12, 4, 1, 37, 52),): 1,
       (datetime.datetime(2021, 12, 4, 1, 2, 8),): 4,
       (datetime.datetime(2021, 12, 4, 1, 5, 14),): 9,
       (datetime.datetime(2021, 9, 28, 0, 43, 51),): 1,
       (datetime.datetime(2021, 12, 4, 1, 0, 13),): 7,
       (datetime.datetime(2021, 12, 4, 1, 9, 59),): 2,
       (datetime.datetime(2021, 12, 3, 17, 25, 46),): 1,
       (datetime.datetime(2021, 12, 4, 1, 9, 54),): 1,
       (datetime.datetime(2021, 9, 24, 1, 14, 30),): 1})
  </code></pre>
  <p>.</p>
  <p>Obj.objects.filter(expired__isnull=False).count()
  returns all the lines (2122) ... .</p>
  <p>Obj.objects.filter(expired=None).count() returns 2087 lines instead of the 35 expected.</p>
  <p>Obj.objects.exclude(expired=None).count() returns 2122, so all the lines.</p>