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

Fix go-gitea#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]`

https://github.com/go-gitea/gitea/blob/ec1feedbf582b05b6a5e8c59fb2457f25d053ba2/services/migrations/gitea_uploader.go#L861-L867

https://github.com/go-gitea/gitea/blob/ec1feedbf582b05b6a5e8c59fb2457f25d053ba2/modules/git/diff.go#L97-L101

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)
 			}