Almanac: Fix PHP 8.1 "strlen(null)" setting Device Property

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.

```
ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/almanac/controller/AlmanacPropertyEditController.php:19]
```

Closes T16418

Test Plan:
* PHP 8.1+
* Make sure Almanac is enabled on http://phorge.localhost/applications/view/PhabricatorAlmanacApplication/
* Go to http://phorge.localhost/almanac/
* Select "Devices"
* Select "Create Device"
* Go to resulting http://phorge.localhost/almanac/device/view/device1/
* Under "Properties", select "Add Property"

Reviewers: O1 Blessed Committers, mainframe98

Reviewed By: O1 Blessed Committers, mainframe98

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16418

Differential Revision: https://we.phorge.it/D26630

src/applications/almanac/controller/AlmanacPropertyEditController.php

···
     $cancel_uri = $object->getURI();
     $property_key = $request->getStr('key');
 
-    if (!strlen($property_key)) {
+    if (!phutil_nonempty_string($property_key)) {
       return $this->buildPropertyKeyResponse($cancel_uri, null);
     } else {
       $error = null;