Check the AdvertiserIdentifier for nil values when implemented by delegate

When the client retrieves the AdvertiserIdentifier in the ADTClient, it does a check for nil values when the delegate has not implemented ADTAdvertiserIdentifer.  The same check is not performed when the delegate HAS implemented the method.

This change extracts that check so that even when the delegate implements the method, a nil value will throw an easily-traceable exception.  Also adjusted the exception message to explicitly state that it resulted from a nil value being passed in.