Clean up a couple more URI alter() calls

Summary:
See https://discourse.phabricator-community.org/t/create-new-phriction-document-fails-with-unhandled-exception-invalidargumentexception/2406.

These weren't obviously nullable from a cursory grep, but are sometimes nullable in practice.

Test Plan: Created, then saved a new Phriction document.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20184