Fix PHP 8.1 "strlen(null)" exceptions setting up Username/Password Authentication 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.

Closes T15383

Test Plan:
Applied these two changes; afterwards "Username/Password" page at `/auth/config/view/1/` gets rendered in web browser.

Additional tests (you need D25231 to complete this test plan):

- Create WordPress Auth; Edit; Disable; Enable (and Disable again to cleanup)
- Create Twitter Auth; Edit; Disable; Enable (and Disable again to cleanup)
- Create Amazon Auth; Edit; Disable; Enable (and Disable again to cleanup)
- Create Asana Auth; Edit; Disable; Enable (and Disable again to cleanup)
- Create Bitbucket Auth; Edit; Disable; Enable (and Disable again to cleanup)
- Create Disqus Auth; Edit; Disable; Enable (and Disable again to cleanup)
- Create Disqus Auth; Edit; Disable Enable (and Disable again to cleanup)
- Create Facebook Auth; Edit; Disable Enable (and Disable again to cleanup)
- Create GitHub Auth; Edit; Disable Enable (and Disable again to cleanup)
- Create JIRA Auth; Edit; Disable; Enable (and Disable again to cleanup)
- Create Google Auth; Edit; Disable; Enable (and Disable again to cleanup)
- Create LDAP Auth; Edit; Disable; Enable (and Disable again to cleanup)
- Create Phorge Auth; Edit; Disable; Enable (and Disable again to cleanup)
- Create Slack Auth; Edit; Disable; Enable (and Disable again to cleanup)
- Create Twitch.tv Auth; Edit; Disable; Enable (and Disable again to cleanup)

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15383

Differential Revision: https://we.phorge.it/D25216

src/applications/auth/controller/config/PhabricatorAuthEditController.php

@@ -99,10 +99,10 @@ final class PhabricatorAuthEditController
 
       if (!$errors) {
         if ($is_new) {
-          if (!strlen($config->getProviderType())) {
+          if (!phutil_nonempty_string($config->getProviderType())) {
             $config->setProviderType($provider->getProviderType());
           }
-          if (!strlen($config->getProviderDomain())) {
+          if (!phutil_nonempty_string($config->getProviderDomain())) {
             $config->setProviderDomain($provider->getProviderDomain());
           }
         }