(#20128) Improve dequeue example to represent two separate resources

The existing spec example for dequeuing events used a single resource and
self-refresh, which didn't accurately reflect the two resource layout in the
original issue #7165 where one resource notifies another.
(#20128) Handle event dequeues when target resource has no events registered

In the case where a resource triggers an event with invalidate_refreshes
enabled (i.e. a service starts), the dequeue process would attempt to
invalidate other events registered for this resource.  If no other
notifications had been received on the resource already, then the hash element
for the resource would be nil.  The resource would then fail:

  Error: /Stage[main]//Service[service]: Could not evaluate: undefined method `[]=' for nil:NilClass
Merge branch 'domcleal-tickets/master/20128-service-dequeue-nil'

* domcleal-tickets/master/20128-service-dequeue-nil:
  (#20128) Handle event dequeues when target resource has no events registered
  (#20128) Improve dequeue example to represent two separate resources

closes #1585