[ios] Don't crash on tap when an AnnotationVIew doesn’t have an annotation

[JesseCrocker]

Fixes #8754
MGLAnnotationView.annotation is marked as nullable, and i think there is a good case to be made that custom annotations views should be able to set their annotation to nil when removed from the map, so enforcing non-null here seems like a bug, and one that there is no easy way to workaround because it is throwing an exception in a call to UIGestureRecognizerDelegate.

[boundsj]

Thanks so much! I'm closing for the reasons stated in #8754 (comment)

The PR #8755 works around a bug caused by this issue but also removes an otherwise valid sanity check. That is, annotation views that are not visible because they are in the reuse queue or removed should have a nil annotation property. The other exceptions that were added as a result of #8139 (review) are all similarly valid. I think we should close #8755 and find another solution.

To that end, I created #9025 to close this issue. I also created #9029 to add an initializer that takes an annotation to facilitate customization of the view based on the annotation model properties.

Let's continue to track this in #8754 and the PRs listed there.