Fix migration panic due to an empty review comment diff (#28334) (#28362)

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]`

ec1feedbf5/services/migrations/gitea_uploader.go (L861-L867)

ec1feedbf5/modules/git/diff.go (L97-L101)

Co-authored-by: Nanguan Lin <70063547+lng2020@users.noreply.github.com>

services/migrations/gitea_uploader.go

@@ -862,7 +862,7 @@ func (g *GiteaLocalUploader) CreateReviews(reviews ...*base.Review) error {
 			line := comment.Line
 			if line != 0 {
 				comment.Position = 1
-			} else {
+			} else if comment.DiffHunk != "" {
 				_, _, line, _ = git.ParseDiffHunkString(comment.DiffHunk)
 			}