ddl: tiny refine the checkForNullValue function. (pingcap#11833)

change "select count(*) from `%s`.`%s` where `%s` is null limit 1"
to "select 1 from `%s`.`%s` where `%s` is null limit 1"

The later would return when the first row meets the condition, thus scanning the whole table is avoided

ddl/column.go

@@ -473,12 +473,12 @@ func (w *worker) doModifyColumn(t *meta.Meta, job *model.Job, newCol *model.Colu
 // checkForNullValue ensure there are no null values of the column of this table.
 // `isDataTruncated` indicates whether the new field and the old field type are the same, in order to be compatible with mysql.
 func checkForNullValue(ctx sessionctx.Context, isDataTruncated bool, schema, table, oldCol, newCol model.CIStr) error {
-	sql := fmt.Sprintf("select count(*) from `%s`.`%s` where `%s` is null limit 1;", schema.L, table.L, oldCol.L)
+	sql := fmt.Sprintf("select 1 from `%s`.`%s` where `%s` is null limit 1;", schema.L, table.L, oldCol.L)
 	rows, _, err := ctx.(sqlexec.RestrictedSQLExecutor).ExecRestrictedSQL(ctx, sql)
 	if err != nil {
 		return errors.Trace(err)
 	}
-	rowCount := rows[0].GetInt64(0)
+	rowCount := len(rows)
 	if rowCount != 0 {
 		if isDataTruncated {
 			return errInvalidUseOfNull