-[MGLMapViewDelegate mapView:viewForAnnotation:] causes assertion failure if upgrading from v3.3.x

[1ec5]

Prior to #5882, -[MGLMapViewDelegate mapView:viewForAnnotation:] was only called for “user-contributed” annotations, never for the user location annotation. Now that it’s additionally called for the user location annotation, the most obvious implementation of this method will trip this assertion unless MGLUserLocationAnnotationView or a user-contributed annotation class is specifically accounted for. Instead of asserting, we should simply ignore the returned annotation view. We can consider making it a hard assertion in a future release.

/cc @friedbunny @frederoni