Fix len

Author: WaVEV

django_mongodb_backend/fields/array.py

@@ -326,7 +326,9 @@ def get_subquery_wrapping_pipeline(self, compiler, connection, field_name, expr)
     def as_mql(self, compiler, connection):
         lhs_mql = process_lhs(self, compiler, connection)
         value = process_rhs(self, compiler, connection)
-        return {"$and": [{"$isArray": lhs_mql}, {"$size": {"$setIntersection": [value, lhs_mql]}}]}
+        return {
+            "$and": [{"$ne": [lhs_mql, None]}, {"$size": {"$setIntersection": [value, lhs_mql]}}]
+        }
 
 
 @ArrayField.register_lookup