Wake Queue#pop(timeout:) waiters when the queue is closed - #9791

The timed poll loop only checked the item count, so the signal from closeInternal woke the waiter, it found the queue still empty, and went back to awaitNanos until the full timeout passed. A closed, empty queue now returns nil at once and passes the wakeup on to the next consumer, as takeInternal already does for the untimed pop.

Run into this while trying to run ONCE Campfire: https://github.com/basecamp/once-campfire/blob/05c5a2c0d72f7fd74d7c2ace23cc123000f956b9/lib/rails_ext/sqlite_wal_checkpoint.rb#L41-L49 and hit the "SQLite WAL checkpointer did not stop within #{timeout}s" error.