[2.0.x] Fixed #28896 -- Reallowed filtering a queryset with GeometryField=None.

Regression in 58da81a5a3.

Backport of da71e4bb08 from master

django/contrib/gis/db/models/fields.py

@@ -167,6 +167,8 @@ class BaseSpatialField(Field):
 
     def get_prep_value(self, value):
         obj = super().get_prep_value(value)
+        if obj is None:
+            return None
         # When the input is not a geometry or raster, attempt to construct one
         # from the given string input.
         if isinstance(obj, GEOSGeometry):

docs/releases/2.0.1.txt

@@ -21,3 +21,5 @@ Bugfixes
 * Fixed a regression in caching of a ``GenericForeignKey`` when the referenced
   model instance uses more than one level of multi-table inheritance
   (:ticket:`28856`).
+
+* Reallowed filtering a queryset with ``GeometryField=None`` (:ticket:`28896`).

tests/gis_tests/geoapp/tests.py

@@ -385,6 +385,9 @@ class GeoLookupTest(TestCase):
         # Puerto Rico should be NULL (it's a commonwealth unincorporated territory)
         self.assertEqual(1, len(nullqs))
         self.assertEqual('Puerto Rico', nullqs[0].name)
+        # GeometryField=None is an alias for __isnull=True.
+        self.assertCountEqual(State.objects.filter(poly=None), nullqs)
+        self.assertCountEqual(State.objects.exclude(poly=None), validqs)
 
         # The valid states should be Colorado & Kansas
         self.assertEqual(2, len(validqs))