Fix a URI construction in remarkup macro/meme rules
Summary: Ref T13250. Some of these parameters may be NULL, and alter() is no longer happy about that.
Test Plan: Ran daemon tasks that happened to render some memes.
Reviewers: amckinley
Reviewed By: amckinley
Maniphest Tasks: T13250
Differential Revision: https://secure.phabricator.com/D20176