Handle isnull when value lhs is a value.

django_mongodb/lookups.py

@@ -1,4 +1,6 @@
+from django.core.exceptions import EmptyResultSet, FullResultSet
 from django.db import NotSupportedError
+from django.db.models.expressions import Value
 from django.db.models.fields.related_lookups import In, MultiColSource, RelatedIn
 from django.db.models.lookups import (
     BuiltinLookup,
@@ -48,6 +50,14 @@ def in_(self, compiler, connection):
 def is_null(self, compiler, connection):
     if not isinstance(self.rhs, bool):
         raise ValueError("The QuerySet value for an isnull lookup must be True or False.")
+    if isinstance(self.lhs, Value):
+        if self.lhs.value is None or (
+            self.lhs.value == "" and connection.features.interprets_empty_strings_as_nulls
+        ):
+            result_exception = FullResultSet if self.rhs else EmptyResultSet
+        else:
+            result_exception = EmptyResultSet if self.rhs else FullResultSet
+        raise result_exception
     lhs_mql = process_lhs(self, compiler, connection)
     return connection.mongo_operators["isnull"](lhs_mql, self.rhs)