Fix a URI construction exception when filtering the Maniphest Burnup chart by project

Summary: See https://discourse.phabricator-community.org/t/filtering-burnup-rate-by-project-produces-exception/2442.

Test Plan: Viewed Maniphest burnup chart, filtered by project: no more URI construction exception.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20207