rustdoc: don't highlight empty elements when they are the URL target

An empty anchor like `<a id="foo"></a>` used as a jump target in docs got
the target border, which shows up as a stray orange bar.

tests/rustdoc-gui/src/lib2/lib.rs

@@ -27,6 +27,7 @@ pub struct Foo {
 
 impl Foo {
     /// Some documentation
+    /// <a id="empty-anchor"></a>
     /// # A Heading
     pub fn a_method(&self) {}
 

tests/rustdoc-gui/target.goml

@@ -33,3 +33,8 @@ call-function: ("check-style", {
     "background": "#fdffd3",
     "border": "#ad7c37",
 })
+
+// An empty anchor must not get the target border.
+go-to: "file://" + |DOC_PATH| + "/lib2/struct.Foo.html#empty-anchor"
+assert: "#empty-anchor:target"
+assert-css: ("#empty-anchor", {"border-right-width": "0px"})