Fix PHP 8.1 "strlen(null)" and "explode()" exceptions which block rendering Administrator Account Creation 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.

Similarly, explode(string $separator, string $string, int $limit) does not accept
passing null instead of an actual string as input parameter either anymore.

Closes T15284

Test Plan: Applied these two changes. Afterwards, admin user account was created and Phorge homepage rendered in web browser on a fresh installation.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15284

Differential Revision: https://we.phorge.it/D25175