ci_run_n_monitor: Allow the upstream format to not exist

When pushing directly to a fork, the upstream branch does not exist and thus
returns an empty string.

Signed-off-by: Corentin Noël <corentin.noel@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27567>

bin/ci/ci_run_n_monitor.py

@@ -435,6 +435,7 @@ if __name__ == "__main__":
                         branch_name,
                     ]).decode('ascii').strip()
 
+                    if tracked_remote:
                         remote_rev = check_output([
                             'git', 'rev-parse', tracked_remote,
                         ]).decode('ascii').strip()