Fix PHP 8.1 "strlen(null)" exception which blocks dragging a task to a Workboard Column

Summary:
On a Workboard without any "Group by <something>", when moving a Task from a Column
to another, in PHP 8.1 there was a crash caused by a null passed to strlen().

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 T15373

Test Plan:
Applied this change and afterwards dragging a Task card to another Column succeeded,
with or without a "Group by <something>" query.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15373

Differential Revision: https://we.phorge.it/D25207