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