I found a workaround by using some JavaScript.
1) First I enclose the the whole list for easier targeting with a div element with an id:
<div id="dpl-list"> <dpl> category = meeting include = #topic </dpl> </div>
2) Then I hide all list items by default and only show those which have an included paragraph using http://www.mediawiki.org/wiki/JQuery_snippets jQuery (e.g. in MediaWiki:Common.css):
$('#dpl-list li').hide(); $('#dpl-list li > p').parent().show();
Et voilà!