[bug](jdbc) fix jdbc insert decimalv3 be core dump (#16353)

Author: zhangstar333

be/src/exec/table_connector.cpp

@@ -279,7 +279,8 @@ Status TableConnector::convert_column_data(const vectorized::ColumnPtr& column_p
     case TYPE_DECIMAL32:
     case TYPE_DECIMAL64:
     case TYPE_DECIMAL128I: {
-        auto val = type_ptr->to_string(*column, row);
+        auto decimal_type = remove_nullable(type_ptr);
+        auto val = decimal_type->to_string(*column, row);
         fmt::format_to(_insert_stmt_buffer, "{}", val);
         break;
     }