ci/github-script/bot: fix infinite labeling cycle

When we recently refactored the code to use the maintainer map for
related labels, we made a mistake: When a PR has no packages with
maintainers returned from eval, the label would internally be set to `0`
instead of `false`.

The code would then go on compare the before and after labels with
strict equality - and assume a difference, because `0 !== false`. Thus,
it seemed like new labels needed to be set, so the PUT request was
actually sent. Of course, the labels were actually the same - when
filtering the labels to be set, the `0` would also be treated as falsy,
so the label would not be set. This would result in no visible change in
the PR, but internall GitHub would replace the `updated_at` timestamp
for that PR - after all we replaced all labels.

Repeatedly updating *all* PRs we're looking at quickly causes problems,
because we are going to look at the same PRs *again* in the next cycle -
essentially causing infinite recursion. The bot became slower and slower
over time, because it had to process more and more PRs each run.

Simply casting this to a proper Boolean, should get us out of the mess
soon.