Fix PHP 8.1 "strlen(null)" warning in XHProf search
Summary: Closes T16339
Test Plan:
* Install the xhprof extension
* Run any arc command with `--xprofile run.json`
* Visit http://phorge.localhost/xhprof/ and upload the xhprof file by drag-and-dropping
* Navigate back to http://phorge.localhost/xhprof/ and see the deprecation in the error log
* Apply this patch and observe it gone
Reviewers: O1 Blessed Committers, aklapper
Reviewed By: O1 Blessed Committers, aklapper
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16339
Differential Revision: https://we.phorge.it/D26497