If PatchURL is empty, skip pull patch download when migrating (Partial #16356) (#16681)

Partial backport #16356

Whilst looking at adding migration support for onedev it has become apparent that gitea would attempt to pull patches on other migration targets even if that PatchURL was empty.

Author: 6543

modules/migrations/gitea_uploader.go

@@ -555,6 +555,9 @@ func (g *GiteaLocalUploader) newPullRequest(pr *base.PullRequest) (*models.PullR
 
 	// download patch file
 	err := func() error {
+		if pr.PatchURL == "" {
+			return nil
+		}
 		// pr.PatchURL maybe a local file
 		ret, err := uri.Open(pr.PatchURL)
 		if err != nil {