Skip requesting diverging commit counts if no branches are listed

When a branch list is displayed in the UI, the diverging commit counts are usually displayed for a subset of branches available in the repository to prevent excessive Gitaly calls using the names parameter. However, when clicking on the different tabs in the branch controller (e.g. Stale, Active), it's possible the list is empty. Since the names parameter is empty, the backend thinks it should return the counts for ALL branches.

We can save considerable I/O in this case by not making the request at all.

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/66200