[persistence] averageSince shouldn't return null

[5iver]

The JavaDoc says that the Items state will be returned if it has not changed since the DateTime specified, but it returns null. This should be changed. Changing the docs would be easy, but I think this would be better functionality.

[cweitkamp]

To find an answer why null is returned you have to dive into this discussion: eclipse-archived/smarthome#2405

As the calculation considers the current value - if known - imo it would make sense to return the current value instead. Maybe @J-N-K can elaborate on this topic.

I think we have to change the JavaDocs anyway because the other method says to return null if no default persistence service is available which is not the case either:

openhab-core/bundles/org.openhab.core.model.persistence/src/org/openhab/core/model/persistence/extensions/PersistenceExtensions.java

Lines 400 to 403 in 2c8de49

	* @return the average state values since <code>timestamp</code>, <code>null</code> if the default persistence
	* service is not available, or the state of the given <code>item</code> if no previous states could be
	* found or if the default persistence service does not refer to an available
	* {@link QueryablePersistenceService}

[J-N-K]

The problem with returning the current value is that this might be wrong. The calculation is essentially the integral of values over time \int_{t0}^{now} y(t) dt divided by the timespan t0-now. If we don't have at least two values (one stored and the current value), we can't calculate the average. I'm not convinced that returning the current state is the correct thing to do in that case.

[J-N-K]

I re-read the old discussion on ESH and I think the consensus reached is still valid. I'll adjust the JavaDoc.