Rename kw::Invalid -> kw::Empty

See https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/Is.20there.20a.20symbol.20for.20the.20empty.20string.3F/near/220054471
for context.

src/librustdoc/html/render/mod.rs

@@ -2086,8 +2086,8 @@ fn item_module(w: &mut Buffer, cx: &Context<'_>, item: &clean::Item, items: &[cl
                 (true, false) => return Ordering::Greater,
             }
         }
-        let lhs = i1.name.unwrap_or(kw::Invalid).as_str();
-        let rhs = i2.name.unwrap_or(kw::Invalid).as_str();
+        let lhs = i1.name.unwrap_or(kw::Empty).as_str();
+        let rhs = i2.name.unwrap_or(kw::Empty).as_str();
         compare_names(&lhs, &rhs)
     }
 
@@ -4206,7 +4206,7 @@ fn print_sidebar(cx: &Context<'_>, it: &clean::Item, buffer: &mut Buffer, cache:
                 ty: \"{ty}\", \
                 relpath: \"{path}\"\
             }};</script>",
-        name = it.name.unwrap_or(kw::Invalid),
+        name = it.name.unwrap_or(kw::Empty),
         ty = it.type_(),
         path = relpath
     );