Fix various "strlen(null)" PHP 8.1 issues on "bin/phd" and "bin/drydock" pathways

Summary: Ref T13676. Ref T13588. Fix some issues that prevent "bin/phd" and "bin/drydock" from executing under PHP 8.1, broadly because null is being passed to strlen().

Test Plan: Ran bin/phd debug task and bin/drydock ... under PHP 8.1.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13676, T13588

Differential Revision: https://secure.phabricator.com/D21795