Conduit: Fix PHP 8.1 "strlen(null)" exceptions in feed.query

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 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/feed/conduit/FeedQueryConduitAPIMethod.php:79]
ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/feed/conduit/FeedQueryConduitAPIMethod.php:84]
```

Closes T16419

Test Plan:
* PHP 8.1+
* Go to http://phorge.localhost/conduit/method/feed.query/
* Press the "Call Method" button
* Go to http://phorge.localhost/conduit/method/feed.query/ and enter values in the `before` and `after` fields
* Press the "Call Method" button

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16419

Differential Revision: https://we.phorge.it/D26631

src/applications/feed/conduit/FeedQueryConduitAPIMethod.php

···
       ->setPageSize($limit);
 
     $after = $request->getValue('after');
-    if (strlen($after)) {
+    if (phutil_nonempty_scalar($after)) {
       $pager->setAfterID($after);
     }
 
     $before = $request->getValue('before');
-    if (strlen($before)) {
+    if (phutil_nonempty_scalar($before)) {
       $pager->setBeforeID($before);
     }