Avoid showing `Failed to change the default wiki branch` if repo has no wiki when saving repo settings (#30329) (#30337)

Backport #30329 by @yp05327

If repo does not have wiki, we should return after save the default wiki
branch into DB.
Or you will always see `Failed to change the default wiki branch` error.

Co-authored-by: yp05327 <576951401@qq.com>

routers/web/repo/wiki_test.go

@@ -226,6 +226,12 @@ func TestWikiRaw(t *testing.T) {
 func TestDefaultWikiBranch(t *testing.T) {
 	unittest.PrepareTestEnv(t)
 
+	// repo with no wiki
+	repoWithNoWiki := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 2})
+	assert.False(t, repoWithNoWiki.HasWiki())
+	assert.NoError(t, wiki_service.ChangeDefaultWikiBranch(db.DefaultContext, repoWithNoWiki, "main"))
+
+	// repo with wiki
 	assert.NoError(t, repo_model.UpdateRepositoryCols(db.DefaultContext, &repo_model.Repository{ID: 1, DefaultWikiBranch: "wrong-branch"}))
 
 	ctx, _ := contexttest.MockContext(t, "user2/repo1/wiki")

services/wiki/wiki.go

@@ -370,6 +370,10 @@ func ChangeDefaultWikiBranch(ctx context.Context, repo *repo_model.Repository, n
 			return fmt.Errorf("unable to update database: %w", err)
 		}
 
+		if !repo.HasWiki() {
+			return nil
+		}
+
 		oldDefBranch, err := gitrepo.GetWikiDefaultBranch(ctx, repo)
 		if err != nil {
 			return fmt.Errorf("unable to get default branch: %w", err)