Fix PHP 8.1/8.2 "strlen(null)" exception in SearchController

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.

EXCEPTION: (RuntimeException) strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261];
PHP message: arcanist(head=master, ref.master=6e4947b55f09), phorge(head=master, ref.master=7bebfa289aa1);
PHP message: #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/search/controller/PhabricatorSearchController.php:16];

Fix T15595

Test Plan: Search something using the main search bar. It should return something (or not) instead to throwing a RuntimeException.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15595

Differential Revision: https://we.phorge.it/D25380