FetchXML - struggling to return records with no child

<fetch mapping="logical" output-format="xml-platform" version="1.0" distinct="true" ><entity name="prorm_project" ><attribute name="prorm_projectid" /><order descending="false" attribute="prorm_projectname" /><link-entity name="prorm_timeentry" from="prorm_rootprojectid" to="prorm_projectid" link-type="outer" alias="ad" ><filter type="and" ><condition attribute="prorm_timeentryid" operator="null" /></filter></link-entity></entity></fetch>