Fix PHP 8.1 "strlen(null)" exceptions creating an OAuth provider

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.

ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/auth/provider/PhabricatorOAuth1AuthProvider.php:163]

ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/auth/provider/PhabricatorOAuth1AuthProvider.php:178]

ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/auth/provider/PhabricatorOAuthAuthProvider.php:147]

Closes T15912

Test Plan: As an admin, create an OAuth provider (such as using Bitbucket) and check the error logs or Dark Console.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15912

Differential Revision: https://we.phorge.it/D25771