Avoid another kw::Empty use.

`sym::dummy` also appears to work.

Author: nnethercote

Diff for: src/librustdoc/clean/utils.rs

@@ -234,7 +234,7 @@ pub(super) fn clean_middle_path<'tcx>(
     args: ty::Binder<'tcx, GenericArgsRef<'tcx>>,
 ) -> Path {
     let def_kind = cx.tcx.def_kind(did);
-    let name = cx.tcx.opt_item_name(did).unwrap_or(kw::Empty);
+    let name = cx.tcx.opt_item_name(did).unwrap_or(sym::dummy);
     Path {
         res: Res::Def(def_kind, did),
         segments: thin_vec![PathSegment {