Return empty url for submodule tree entries (go-gitea#23043)

Close go-gitea#22614.

Refer to [Github's
API](https://docs.github.com/en/rest/git/trees?apiVersion=2022-11-28#get-a-tree),
if a tree entry is a submodule, its url will be an empty string.

---------

Co-authored-by: delvh <leon@kske.dev>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>

services/repository/files/tree.go

@@ -86,6 +86,11 @@ func GetTreeBySHA(ctx context.Context, repo *repo_model.Repository, gitRepo *git
  if entries[e].IsDir() {
  copy(treeURL[copyPos:], entries[e].ID.String())
  tree.Entries[i].URL = string(treeURL)
+ } else if entries[e].IsSubModule() {
+ // In Github Rest API Version=2022-11-28, if a tree entry is a submodule,
+ // its url will be returned as an empty string.
+ // So the URL will be set to "" here.
+ tree.Entries[i].URL = ""
  } else {
  copy(blobURL[copyPos:], entries[e].ID.String())
  tree.Entries[i].URL = string(blobURL)