Trigger feedback event even when there are no items

Triggers the feedback service's `feedback` event with an empty result set when initialized with `{batchResults: true, interval: 0}`. 

When used this way, the feedback service runs once and disconnects after triggering a single `feedback` event. However, if there were no results, there was no way to know that the request succeeded.

We bumped into this problem while using the `Feedback` class to fetch the feedback as a response to a query from another service, when there was just no events from the feedback object at all for empty result sets. When used as a continuous service (non zero interval) it makes sense to not trigger the events for empty sets, but when fired for one query, it's more reasonable to trigger the empty event.