Auto merge of #86765 - cuviper:fuse-less-specialized, r=joshtriplett

Make the specialized Fuse still deal with None

Fixes #85863 by removing the assumption that we'll never see a cleared iterator in the `I: FusedIterator` specialization. Now all `Fuse` methods check for the possibility that `self.iter` is `None`, and the specialization only avoids _setting_ that to `None` in `&mut self` methods.

Author: bors