R1:ac4c95169973

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…