ParseMysqlGTIDSet should handle str is empty (go-mysql-org#135)

Author: IANTHEREAL

mysql/mysql_gtid.go

@@ -291,11 +291,13 @@ type MysqlGTIDSet struct {
 
 func ParseMysqlGTIDSet(str string) (GTIDSet, error) {
 s := new(MysqlGTIDSet)
-
+s.Sets = make(map[string]*UUIDSet)
+if str == "" {
+return s, nil
+}
+
 sp := strings.Split(str, ",")
 
-s.Sets = make(map[string]*UUIDSet, len(sp))
-
 //todo, handle redundant same uuid
 for i := 0; i < len(sp); i++ {
 if set, err := ParseUUIDSet(sp[i]); err != nil {