Fix PHP 8.1 "strlen(null)" exceptions which block rendering Log In page after user logout

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 T15384

Test Plan: Applied these four changes; logged in again; logged out again; finally saw "Log In" page with "Username or Email" and "Password" field on /auth/loggedout/ rendered in web browser.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

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

Maniphest Tasks: T15384

Differential Revision: https://we.phorge.it/D25217