fix submodule is nil panic (#23588) (#23601)

Backport #23588 by @viletyy

#23587  
submodule path is nil 
It is panic a nil error

Co-authored-by: 来自村里的小螃蟹 <yystopf@163.com>

services/repository/files/content.go

@@ -214,7 +214,9 @@ func GetContents(ctx context.Context, repo *repo_model.Repository, treePath, ref
 		if err != nil {
 			return nil, err
 		}
-		contentsResponse.SubmoduleGitURL = &submodule.URL
+		if submodule != nil && submodule.URL != "" {
+			contentsResponse.SubmoduleGitURL = &submodule.URL
+		}
 	}
 	// Handle links
 	if entry.IsRegular() || entry.IsLink() {