AccountService: Implements default account failsafe

Implements one of the fixes outlined in issue #2637.

/cc @sendhil + @astralbodies + @koke 

@sendhil sorry, this should have gone elsewhere, most likely. One of the unit tests was crashing violently, when `self.currentTopic` was nil.

By using the `dictionaryWithObjectsAndKey` API, worst case scenario, if the topic is nil, we get an empty dictionary. (The literal breaks with a nil!).