R1:eb2be08f9f0e

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.