kernel: poll: Allow 0 events for submitted work

Change:
    commit cc6317d7
    Author: Jukka Rissanen <jukka.rissanen@linux.intel.com>
    Date:   Fri Nov 1 14:03:32 2019 +0200

        kernel: poll: Allow 0 event input

Allows `k_poll` to be user with 0 events, which is useful for allowing just
a sleep without having to create artificial events.

Allow the same for `k_work_submit_to_queue()` and `k_work_submit()`.

Signed-off-by: David Brown <david.brown@linaro.org>