Fix submodule parsing when the gitmodules is missing (#35109) (#35118)

Backport #35109

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

services/repository/files/tree.go

@@ -178,7 +178,9 @@ func newTreeViewNodeFromEntry(ctx context.Context, repoLink string, renderedIcon
 		} else if subModule != nil {
 			submoduleFile := git.NewCommitSubmoduleFile(repoLink, node.FullPath, subModule.URL, entry.ID.String())
 			webLink := submoduleFile.SubmoduleWebLinkTree(ctx)
-			node.SubmoduleURL = webLink.CommitWebLink
+			if webLink != nil {
+				node.SubmoduleURL = webLink.CommitWebLink
+			}
 		}
 	}