Use JSON type for JSON null expressions

Keep generated JSON null expressions typed with the JSON type mapping CLR type instead of object, so COALESCE and JSON setter expressions stay well-typed.

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

Author: roji

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

@@ -1444,7 +1444,7 @@ private SqlExpression JsonNull(RelationalTypeMapping jsonTypeMapping)
         => new SqlUnaryExpression(
             ExpressionType.Convert,
             _sqlExpressionFactory.Constant("null"),
-            typeof(object),
+            jsonTypeMapping.ClrType,
             jsonTypeMapping);
 
     private SqlExpression CoalesceToJsonNull(SqlExpression value, RelationalTypeMapping jsonTypeMapping)