Fix Timestamp.IsZero (go-gitea#21593)

Our implementation of `IsZero` can't work. An "empty" timestamp (= 0)
calls `time.Unix(int64(ts), 0).IsZero()` which is always `false`. Only
`time.Time{}.IsZero()` is `true`.
We call this method ~~only at one place~~ and there the value
(`UpdatedUnix`) should be always != 0 so this PR may not have
consequences.

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
 }