Fix invalid bounds string generation in rustdoc

Fixes #58737.

Very weird and I'm not sure this is the best fix around. However, trying to fix it beforehand seems overly complicated compared to the gain (in `clean`, it wouldn't change anything since we **have to** return something so that wouldn't work, and in `hir`, I'm afraid I'd break something else for very little gain).

Also, I wasn't able to make a small code to reproduce the issue. The only way to test is to document `crossbeam` directly and check the `Scope` struct...

r? @QuietMisdreavus