Fix PHP 8.1 "strlen(null)" exception which blocks rendering most applications' Configure pages

Summary:
strlen() was used in Phabricator to check if a generic value is a non-empty string.
This behavior is deprecated since PHP 8.1. Phorge adopts phutil_nonempty_string() as a replacement.

Note: this may highlight other absurd input values that might be worth correcting
instead of just ignoring. If phutil_nonempty_string() throws an exception in your
instance, report it to Phorge to evaluate and fix that specific corner case.

Added also a small inline documentation to better leave what I find.

Closes T15358

Test Plan:
Applied this change change and way more applications' Configure pages finally rendered in web browser.
For example, /applications/view/PhabricatorAuditApplication/ is now correctly rendered.
(However, some pages expose followup exceptions to be handled in separate tasks.)

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15358

Differential Revision: https://we.phorge.it/D25197