Fix PHP 8.1 "strlen(null)" exception[s] which blocks rendering External Editor user settings page

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.

Closes T15310

Test Plan:
Applied this change (on top of D25144, D25145, D25146, D25147, D25151,
D25152, D25153) and /settings/panel/editor/ correctly rendered in web browser.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15310

Differential Revision: https://we.phorge.it/D25160