Fix some "URI->alter(X, null)" callsites

Summary:
Ref T13250. This internally calls replaceQueryParam(X, null) now, which fatals if the second parameter is null. I hit these legitimately, but I'll look for more callsites and follow up by either allowing this, removing alter(), fixing the callsites, or some combination.

(I'm not much of a fan of alter().)

Test Plan: Browsing a paginated list no longer complains about URI construction.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13250

Differential Revision: https://secure.phabricator.com/D20162