ActiveJob: allow empty queue names

As discussed in #17195, Que uses blank queue names by default for
performance reasons. At the very least, ActiveJob should allow users to
choose "" as their queue name. This adds a failing test to show that
this doesn't currently work, and then adds a patch to allow this behavior.
Right, because you needed to do a `presence` check which isn't available on Symbol.

But since empty strings/symbols should be OK now, the `&& part_name.to_s` isn't needed anymore.