R1:000bccf8e4f7

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…