Accommodate PHP 7.4 changes to certain "preg_match_all()" calls

Summary:
Ref T13518. The result format of this call changed in PHP 7.4, which causes us to emit "-1" matches because "-1" survives array_filter().

Filter results in a way that should survive both result formats.

Test Plan: Ran arc unit --everything under PHP 7.4.

Maniphest Tasks: T13518

Differential Revision: https://secure.phabricator.com/D21173