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