Add check for empty set when dropping indexes during migration (#8471)

* Add check for empty set when dropping indexes during migration

models/migrations/migrations.go

@@ -404,11 +404,13 @@ func dropTableColumns(sess *xorm.Session, tableName string, columnNames ...strin
 		}
 		for _, index := range res {
 			indexName := index["column_name"]
+			if len(indexName) > 0 {
 				_, err := sess.Exec(fmt.Sprintf("DROP INDEX `%s` ON `%s`", indexName, tableName))
 				if err != nil {
 					return err
 				}
 			}
+		}
 
 		// Now drop the columns
 		cols := ""