Fix a lint renderering issue

Summary: Currently there is an issue when renderering linter messages with ArcanistConsoleLintRenderer if $message->getChar() returns 0 (i.e. the value is unset).

Test Plan:
Before

After

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11319