Initialize array properties on standalone Swift RLMObject subclasses

With Swift RLMObject subclasses, RLMArray properties start out as placeholder array objects rather than nil, so handle converting them into proper standalone arrays on first use.

@alazier 
I think it would be cleaner to do this in the initializer for RLMArray. We would just need to check if the class is RLMArray and not one of the variants, as the base is the only one which needs a standalone backing array.