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

Follow up #35096, fix #35095, fix #35115 and add more tests

The old code used some fragile behaviors which depend on the "nil"
receiver. This PR should be a complete fix for more edge cases.

services/repository/files/tree.go

@@ -174,7 +174,9 @@ 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
+			}
 		}
 	}