Don't allow empty post content

Parsing the post content makes it non-empty (`<t></t>`), so we don't
parse it if it is empty. Also delete a created discussion if the first
post was invalid.

closes flarum/core#224

src/Core/Discussions/Commands/StartDiscussionHandler.php

@@ -6,6 +6,7 @@ use Illuminate\Contracts\Bus\Dispatcher;
 use Flarum\Core\Discussions\Discussion;
 use Flarum\Core\Posts\Commands\PostReply;
 use Flarum\Core\Support\DispatchesEvents;
+use Exception;
 
 class StartDiscussionHandler
 {
@@ -57,9 +58,15 @@ class StartDiscussionHandler
 
         // Now that the discussion has been created, we can add the first post.
         // We will do this by running the PostReply command.
+        try {
             $post = $this->bus->dispatch(
                 new PostReply($discussion->id, $actor, $data)
             );
+        } catch (Exception $e) {
+            $discussion->delete();
+
+            throw $e;
+        }
 
         // Before we dispatch events, refresh our discussion instance's
         // attributes as posting the reply will have changed some of them (e.g.

src/Core/Posts/CommentPost.php

@@ -142,7 +142,7 @@ class CommentPost extends Post
      */
     public function setContentAttribute($value)
     {
-        $this->attributes['content'] = static::$formatter->parse($value, $this);
+        $this->attributes['content'] = $value ? static::$formatter->parse($value, $this) : null;
     }
 
     /**