Correct some Arcanist behaviors under PHP8.1

Summary:
Ref T13588. See that task for discussion.

Improve behavior under PHP8.1, particularly the deprecation warning raised by calling strlen(null).

Test Plan:

• Ran arc help, arc branches, arc diff, etc., under PHP 8.1 and PHP 7.4.
• Created this change with PHP8.1.

Maniphest Tasks: T13588

Differential Revision: https://secure.phabricator.com/D21740