Remove leading whitespace if no prefix is configured for Asana sync

Summary: Ref T2852. If the prefix is removed by configuration, we'll incorrectly leave a leading space. Trim any leading whitespace off.

Test Plan: Ran bin/feed republish to sync an object to Asana

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6575