fix DatabaseOperations.adapt_datetimefield_value() crash on null DateTimeField

Author: timgraham

.github/workflows/test-python.yml

@@ -67,6 +67,7 @@ jobs:
       - name: Run tests
         run: >
           python3 django_repo/tests/runtests.py --settings mongodb_settings -v 2
+          auth_tests.test_models.UserManagerTestCase
           backends.base.test_base.DatabaseWrapperTests
           basic
           dates

django_mongodb/operations.py

@@ -11,7 +11,7 @@ class DatabaseOperations(BaseDatabaseOperations):
     compiler_module = "django_mongodb.compiler"
 
     def adapt_datetimefield_value(self, value):
-        if not settings.USE_TZ and timezone.is_naive(value):
+        if not settings.USE_TZ and value is not None and timezone.is_naive(value):
             value = timezone.make_aware(value)
         return value