LibWeb: Avoid dereferencing an empty optional URL

Here, "null" means the empty optional. We don't need to also check if
the URL is valid; the url will be null if it was originally invalid.

Libraries/LibWeb/HTML/HTMLHyperlinkElementUtils.cpp

@@ -427,7 +427,7 @@ String HTMLHyperlinkElementUtils::href() const
         return String {};
 
     // 4. Otherwise, if url is null, return this element's href content attribute's value.
-    if (!url->is_valid())
+    if (!url.has_value())
         return href_content_attribute.release_value();
 
     // 5. Return url, serialized.