Fix PHP 8.1 "strlen(null)" exception which blocks editing a Dashboard Tab Panel

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.

Closes T15345

Test Plan: After applying this change, the "Edit Panel" page at `/dashboard/panel/edit/2/` correctly rendered in the web browser.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15345

Differential Revision: https://we.phorge.it/D25189

src/applications/dashboard/controller/panel/PhabricatorDashboardPanelEditController.php

···
     // editing.
 
     $context_phid = $request->getStr('contextPHID');
-    if (strlen($context_phid)) {
+    if (phutil_nonempty_string($context_phid)) {
       $context = id(new PhabricatorObjectQuery())
         ->setViewer($viewer)
         ->withPHIDs(array($context_phid))