Fix migration panic due to an empty review comment diff (#28334)

[GiteaBot]

Backport #28334 by @lng2020

Fix #28328

func (p *PullRequestComment) GetDiffHunk() string {
	if p == nil || p.DiffHunk == nil {
		return ""
	}
	return *p.DiffHunk
}

This function in the package go-github may return an empty diff. When it's empty, the following code will panic because it access ss[1]

gitea/services/migrations/gitea_uploader.go

Lines 861 to 867 in ec1feed

	for _, comment := range review.Comments {
	line := comment.Line
	if line != 0 {
	comment.Position = 1
	} else {
	_, _, line, _ = git.ParseDiffHunkString(comment.DiffHunk)
	}

gitea/modules/git/diff.go

Lines 97 to 101 in ec1feed

	func ParseDiffHunkString(diffhunk string) (leftLine, leftHunk, rightLine, righHunk int) {
	ss := strings.Split(diffhunk, "@@")
	ranges := strings.Split(ss[1][1:], " ")
	leftRange := strings.Split(ranges[0], ",")
	leftLine, _ = strconv.Atoi(leftRange[0][1:])