Don't run linter when there are no overall changes

I had a merge request with no overall change: !1621 (merged).

This caused the lint job to fail: https://gitlab.torproject.org/tpo/applications/tor-browser/-/jobs/1091006.

I think the problem was that .gitlab/ci/jobs/lint/helpers.py --get-changed-files returned an empty list. Which meant that ./mach lint -v was run without any arguments. As a result, I think it might have tried to lint everything, including the content of .cache/.

I think you might be able to fix this by adding the --no-run-if-empty argument to xargs.