Fix PHP 8.1 "strlen(null)" exceptions which block adding Dashboard to Project Menu

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 T15339

Test Plan: Applied these four changes and `/dashboard/install/1/project/` finally rendered in web browser.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15339

Differential Revision: https://we.phorge.it/D25184

src/applications/dashboard/install/PhabricatorDashboardObjectInstallWorkflow.php

···
       $target_identifier = head($target_tokens);
     }
 
-    if (!strlen($target_identifier)) {
+    if (!phutil_nonempty_string($target_identifier)) {
       $target_identifier = $request->getStr('target');
     }
 
-    if (!strlen($target_identifier)) {
+    if (!phutil_nonempty_string($target_identifier)) {
       $target_identifier = $this->getMode();
     }
 
     $target = null;
-    if (strlen($target_identifier)) {
+    if (phutil_nonempty_string($target_identifier)) {
       $targets = array();
 
       if (ctype_digit($target_identifier)) {
···
     }
 
     $errors = array();
-    if (strlen($target_identifier)) {
+    if (phutil_nonempty_string($target_identifier)) {
       if (!$target) {
         $errors[] = pht('Choose a valid object.');
       } else if (!$can_edit) {