Fix PHP 8.1 "strlen(null)" exception shown when sending SMS Test Message to Contact Number

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 T15382

Test Plan:
Applied this change and after performing steps to send a test message again, /mail/detail/107/ page got rendered correctly
and I can see the "Status Details" with stuff like "None" or "No mailers are configured."

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15382

Differential Revision: https://we.phorge.it/D25215