resolve: Fix resolution of empty paths passed from rustdoc

src/test/rustdoc-ui/intra-doc/global-path.rs

@@ -0,0 +1,8 @@
+// Doc link path with empty prefix that resolves to "extern prelude" instead of a module.
+
+// check-pass
+// edition:2018
+
+/// [::Unresolved]
+//~^ WARN unresolved link to `::Unresolved`
+pub struct Item;

src/test/rustdoc-ui/intra-doc/global-path.stderr

@@ -0,0 +1,10 @@
+warning: unresolved link to `::Unresolved`
+  --> $DIR/global-path.rs:6:6
+   |
+LL | /// [::Unresolved]
+   |      ^^^^^^^^^^^^ no item named `` in scope
+   |
+   = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default
+
+warning: 1 warning emitted
+