Fix a PHP 8.1 issue in lint rendering

Summary: Ref T13588. The lint renderer has some "strlen(null)" issues; correct them.

Test Plan: Hit lint messages under PHP 8.1.

Reviewers: 0

Subscribers: jeremy.norris, vostok4, vhbit, jaydiablo, 0

Maniphest Tasks: T13588

Differential Revision: https://secure.phabricator.com/D21743