fix(release): allow publishing drafts without target #35569 (#38800)

### Issue

Fully publishing a draft release fails when the release does not have a
target set.

A draft release can be created without a target because the Git tag is
not created while the release remains a draft. However, when the draft
is published, Gitea attempts to resolve the release target to create the
tag. If the target is empty, the publish operation fails.

### Solution

Use the repository's default branch as the target when a draft release
is published without an explicitly configured target.

Also add a regression test to verify that a draft release without a
target can be successfully published.

Fixes #35569

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/release/release.go

@@ -102,7 +102,8 @@ func createTag(ctx context.Context, gitRepo *git.Repository, rel *repo_model.Rel
 				}
 			}
 
-			commit, err := gitRepo.GetCommit(ctx, rel.Target)
+			target := util.IfZero(rel.Target, rel.Repo.DefaultBranch)
+			commit, err := gitRepo.GetCommit(ctx, target)
 			if err != nil {
 				return false, err
 			}

services/release/release_test.go

@@ -228,7 +228,7 @@ func TestRelease_Update(t *testing.T) {
 		PublisherID:  user.ID,
 		Publisher:    user,
 		TagName:      "v1.1.2",
-		Target:       "master",
+		Target:       "",
 		Title:        "v1.1.2 is released",
 		Note:         "v1.1.2 is released",
 		IsDraft:      true,