Rollup merge of #137769 - compiler-errors:empty-unsafe-fmt, r=ytmimi

Do not yeet `unsafe<>` from type when formatting unsafe binder

Unsafe binders are types like `unsafe<'a, 'b> Ty<'a, 'b>`. However, users can also specify unsafe binder types with no bound vars, like `unsafe<> Ty`.

When I added nightly formatting for unsafe binders, I didn't consider this, so on nightly we are stripping the `unsafe<>` part, which gives us back `Ty` which is a different type!

This PR fixes that.

r? ``@ytmimi``

Author: matthiaskrgr