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 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```

compiler/rustc_middle/src/ty/assoc.rs

@@ -246,6 +246,8 @@ impl AssocItems {
     }
 
     /// Returns an iterator over all associated items with the given name, ignoring hygiene.
+    ///
+    /// Panics if `name.is_empty()` returns `true`.
     pub fn filter_by_name_unhygienic(
         &self,
         name: Symbol,