Untested behaviour when decide_worker returns None

Accidentally, while I was developing https://github.com/dask/distributed/pull/8135 , coveralls highlighted that there are no tests for what happens when
`transition_no_worker_processing` or `transition_queued_processing` fail to transition the task to processing because `decide_worker_non_rootish` or `decide_worker_rootish_queuing_enabled` (respectively) returned None.

There may be other use cases.
It feels too big of a corner case, with too much potential for deadlocking, to leave it untested.

CC @fjetter @hendrikmakait