Fix JSON null type mapping on hotfix

Assign a string type mapping to the JSON null literal operand before casting it to the JSON mapping, avoiding EF10 type-mapping postprocessor failures in ExecuteUpdate JSON tests.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: roji

src/EFCore.PG/Query/Internal/NpgsqlQueryableMethodTranslatingExpressionVisitor.cs

@@ -1443,7 +1443,7 @@ protected override bool TrySerializeScalarToJson(
     private SqlExpression JsonNull(RelationalTypeMapping jsonTypeMapping)
         => new SqlUnaryExpression(
             ExpressionType.Convert,
-            _sqlExpressionFactory.Constant("null"),
+            _sqlExpressionFactory.Constant("null", _typeMappingSource.FindMapping(typeof(string))),
             jsonTypeMapping.ClrType,
             jsonTypeMapping);