Fix error when an intra doc link is trying to resolve an empty associated item
Fixes https://github.com/rust-lang/rust/issues/140026.
Assigning ```@nnethercote``` since they're the one who wrote the initial change.
I updated rustdoc code instead of compiler's because I think it makes more sense that the caller ensures on their side that the name they're looking for isn't empty.
r? ```@nnethercote```
96ac7d8b5e