Fix PHP 8.1 "strlen(null)" exception which blocks blog rendering

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 T15356

Test Plan: Applied this change change on top of D25194 and `/phame/blog/view/1/` 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: T15356

Differential Revision: https://we.phorge.it/D25195

src/applications/phame/controller/PhameLiveController.php

@@ -87,7 +87,7 @@ abstract class PhameLiveController extends PhameController {
     $this->isExternal = $is_external;
     $this->isLive = $is_live;
 
-    if (strlen($post_id)) {
+    if (phutil_nonempty_string($post_id)) {
       $post_query = id(new PhamePostQuery())
         ->setViewer($viewer)
         ->needHeaderImage(true)