kernel: work: check handler when submit to queue

Assert that the handler of a work is not NULL when submitting
it to the queue. This allows early detection of the
code that is submitting a non-NULL work with NULL handler to
the work queue (where it happens), rather than right before the
work item get executed in the queue (when it happens).

Signed-off-by: Yong Cong Sin <ycsin@meta.com>