Add unit test.

Author: WaVEV

django_mongodb/features.py

@@ -350,7 +350,6 @@ class DatabaseFeatures(BaseDatabaseFeatures):
         },
         "Mongodb's Null behaviour is different from sql's": {
             "model_fields.test_jsonfield.TestQuerying.test_none_key_and_exact_lookup",
-            # "model_fields.test_jsonfield.TestQuerying.test_isnull_key",
             "model_fields.test_jsonfield.TestSaveLoad.test_json_null_different_from_sql_null",
             "model_fields.test_jsonfield.TestQuerying.test_none_key",
             "model_fields.test_jsonfield.TestQuerying.test_lookup_exclude",