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```

tests/rustdoc-ui/intra-doc/empty-associated-items.rs

@@ -0,0 +1,8 @@
+// This test ensures that an empty associated item will not crash rustdoc.
+// This is a regression test for <https://github.com/rust-lang/rust/issues/140026>.
+
+#[deny(rustdoc::broken_intra_doc_links)]
+
+/// [`String::`]
+//~^ ERROR
+pub struct Foo;

tests/rustdoc-ui/intra-doc/empty-associated-items.stderr

@@ -0,0 +1,14 @@
+error: unresolved link to `String::`
+  --> $DIR/empty-associated-items.rs:6:7
+   |
+LL | /// [`String::`]
+   |       ^^^^^^^^ the struct `String` has no field or associated item named ``
+   |
+note: the lint level is defined here
+  --> $DIR/empty-associated-items.rs:4:8
+   |
+LL | #[deny(rustdoc::broken_intra_doc_links)]
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+