Fix null constrained

Author: michaelstaib

src/HotChocolate/MongoDb/src/Data/Driver/AndFilterDefinition.cs

@@ -78,7 +78,7 @@ private static void AddClause(BsonDocument document, BsonElement clause)
                     ? clauseValue.GetElement(0).Name
                     : null;
                 if (clauseValue.Names.Any(op => existingClauseValue.Contains(op)) ||
-                    __operatorsThatCannotBeCombined.Contains(clauseOperator))
+                    (clauseOperator is not null && __operatorsThatCannotBeCombined.Contains(clauseOperator)))
                 {
                     PromoteFilterToDollarForm(document, clause);
                 }