Fix PHP 8.1 "strlen(null)" exception calling Conduit's user.whoami

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.

```
EXCEPTION: (RuntimeException) strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=master, ref.master=97e163187418), phorge(head=master, ref.master=7bbd657c19c4)
#0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/conduit/controller/PhabricatorConduitAPIController.php:686]
```

Closes T15469

Test Plan: After applying this change, `/api/user.whoami` correctly renders the "Method Call Result"

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

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

Maniphest Tasks: T15469

Differential Revision: https://we.phorge.it/D25292

src/applications/conduit/controller/PhabricatorConduitAPIController.php

···
     // Otherwise, look for a single parameter called 'params' which has the
     // entire param dictionary JSON encoded.
     $params_json = $request->getStr('params');
-    if (strlen($params_json)) {
+    if (phutil_nonempty_string($params_json)) {
       $params = null;
       try {
         $params = phutil_json_decode($params_json);