Fix `Timestamp.IsZero` (#21593) (#21604)

Backport of #21593

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/timeutil/timestamp.go

@@ -103,5 +103,5 @@ func (ts TimeStamp) FormatDate() string {
 
 // IsZero is zero time
 func (ts TimeStamp) IsZero() bool {
-	return ts.AsTimeInLocation(time.Local).IsZero()
+	return int64(ts) == 0
 }