Rollup merge of #140052 - GuillaumeGomez:fix-140026, r=nnethercote

Fix error when an intra doc link is trying to resolve an empty associated item

Fixes rust-lang/rust#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```

Author: ChrisDenton