Fix notice in content search plugin

During #5276 I changed from catslug to catid in the call to ContentHelperRoute::getArticleRoute(). Unfortunately, we don't retrieve the catid from the database. That is why we get a notice here.

### Testinstructions
Search something in your Joomla installation and make sure that you have the content search plugin enabled. Notice the notice in the output for every content link.
Apply the change, see the notice disappear.

Thanks @shur for spotting this bug.