Fix symbol handling in symbol query and IRC "Where is x?" handler

Summary: If a symbol's project has no linked repository, we currently explode. Instead, decline to generate a URI and fall back gracefully.

Test Plan: https://secure.phabricator.com/chatlog/channel/%23phabricator/?at=22345

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1465

Differential Revision: https://secure.phabricator.com/D2948