You can save the array returned by the filter, and use its length to toggle ng-show
:
<li ng-repeat="item in (filteredContent = (content | filter:{'data':'category1'}))">
...
<li ng-show="filteredContent.length == 0">No content available</li>
In this demo http://plnkr.co/7kMjojVAkCkTR8XQlunB, try changing the filter so that nothing is matched.