Fix Diviner strlen(null) error when clicking on a link

Summary:
Fix issue whereby clicking on a link in Diviner (eg https://my.phorge.site/diviner/find/?name=Differential_User_Guide&type=article&jump=1) results in a strlen(null) error under PHP 8.1

Fixes T15635

Test Plan:
* Go into the Diviner differential page /book/phorge/article/differential/
* Click on the link to Differential User Guide: Inline Comments

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15635

Differential Revision: https://we.phorge.it/D25433

src/applications/diviner/controller/DivinerFindController.php

@@ -32,7 +32,7 @@ final class DivinerFindController extends DivinerController {
     }
 
     $context = $request->getStr('context');
-    if (strlen($context)) {
+    if (phutil_nonempty_string($context)) {
       $query->withContexts(array($context));
     }