Fix undefined var in recent change for new comment

With the recent changes module, adding a recent change for a new comment
caused the warning "Undefined variable $comment_thread" if the comment
was not a reply. Use `isset()` instead of `!== NULL` to check for reply
depth, avoiding the warning when $comment_thread is not set.

modules/feature-comments.php

@@ -134,7 +134,7 @@ register_module([
 					"timestamp" => time(),
 					"page" => $env->page,
 					"user" => $env->user,
-					"reply_depth" => $comment_thread !== null ? count($comment_thread) : 0,
+					"reply_depth" => isset($comment_thread) ? count($comment_thread) : 0,
 					"comment_id" => $new_comment->id
 				]);
 			}