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.

src/librustdoc/html/static/css/rustdoc.css

@@ -1952,7 +1952,7 @@ instead, we check that it's not a "finger" cursor.
 	}
 }
 
-:target:not([data-nosnippet]) {
+:target:not([data-nosnippet]):not(:empty) {
 	background-color: var(--target-background-color);
 	border-right: 3px solid var(--target-border-color);
 }