Add isnull expression in partial index.

Author: WaVEV

django_mongodb/base.py

@@ -126,6 +126,10 @@ def _isnull_operator(a, b):
         "lt": lambda a, b: {a: {"$lt": b}},
         "lte": lambda a, b: {a: {"$lte": b}},
         "in": lambda a, b: {a: {"$in": b}},
+        # Partial indexes do not support the $not expression.
+        # Operators like $exists: False, $ne, and $nor cannot be used in partial indexes.
+        # So, indexes on null values (isnull=True|False) can't be implemented.
+        "isnull": lambda a, b: {},  # noqa: ARG005
     }
 
     display_name = "MongoDB"