Fix PHP 8.1 "strlen(null)" exception in PhabricatorMailSMTPAdapter when sending email
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.
```
ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/htdocs/phorge/phorge/src/applications/metamta/adapter/PhabricatorMailSMTPAdapter.php:65]
```
Closes T15857
Test Plan: Send Welcome email via 'Manage' user screen and Run `./bin/phd log` afterwards to validate email action
Reviewers: O1 Blessed Committers, speck
Reviewed By: O1 Blessed Committers, speck
Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15857
Differential Revision: https://we.phorge.it/D25692