Fix PHP 8.1 "strlen(null)" and "array_slice(null)" exceptions which block typeahead completion proposals

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.

array_slice() no longer accepts passing null as a parameter.
This behavior is deprecated since PHP 8.1. Adding an if clause; not using a
Null Coalescing Operator (PHP 7+) as Phorge currently still supports PHP 5.5.

Closes T15321

Test Plan:
Applied these two changes on top of D25147. Afterwards, typeahead autocompletion
proposal dropdowns for the three fields "Assigned To", "Subscribers", "Tags" got
displayed in web browser.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15321

Differential Revision: https://we.phorge.it/D25170