Allow for null actor (and probably others) in LearningResourceStore xapi service constructor

Relates to some other work in KNL-1451 and KNL-1454, the xapi constructor doesn't allow any of the fields to be null to be null, but it's trivial to look up the actor from the event when it's processing it if it's not available. It's also possible to look up every other value even though they might not return anything.

I don't think there should be an api requirement for non-null and it should look it up

If it's not null it means there's a circular dependency on KNL-1451 that allows you to add an LRS_Statement to newEvent.. But you can't create an LRS_Statement without an actor that requires the event first because you need to call the convenience method of getEventActor(event) on the service to get it.

I'm just going to modify this so the service if you register or process events will attempt to look these up if needed.