Fix migrate storage bug (#28830) (#28867)

Backport #28830 by @lunny

Fix #28728

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

cmd/migrate_storage.go

@@ -110,6 +110,9 @@ func migrateLFS(ctx context.Context, dstStorage storage.ObjectStorage) error {
 
 func migrateAvatars(ctx context.Context, dstStorage storage.ObjectStorage) error {
 	return db.Iterate(ctx, nil, func(ctx context.Context, user *user_model.User) error {
+		if user.CustomAvatarRelativePath() == "" {
+			return nil
+		}
 		_, err := storage.Copy(dstStorage, user.CustomAvatarRelativePath(), storage.Avatars, user.CustomAvatarRelativePath())
 		return err
 	})
@@ -117,6 +120,9 @@ func migrateAvatars(ctx context.Context, dstStorage storage.ObjectStorage) error
 
 func migrateRepoAvatars(ctx context.Context, dstStorage storage.ObjectStorage) error {
 	return db.Iterate(ctx, nil, func(ctx context.Context, repo *repo_model.Repository) error {
+		if repo.CustomAvatarRelativePath() == "" {
+			return nil
+		}
 		_, err := storage.Copy(dstStorage, repo.CustomAvatarRelativePath(), storage.RepoAvatars, repo.CustomAvatarRelativePath())
 		return err
 	})