Fix PHP 8.1 "strlen(null)" exception which blocks task Bulk Editor page

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 T15391

Test Plan:
Applied this change and afterwards, "Bulk Editor" page on /maniphest/bulk/ rendered correctly.

To test that:

• visit a list of some Tasks (like /maniphest/)
• filter something (this can be done in multiple ways)
• select Tasks
• click on Bulk Edit Selected Tasks

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15391

Differential Revision: https://we.phorge.it/D25223