Squashed commit of the following:

commit 41ab44ed1d86a1d0788089cf2d79e3c9ab8e2ba6
Author: Jonny Barnes <jonny@jonnybarnes.uk>
Date:   Wed Aug 9 20:58:13 2017 +0100

    Add a test for a swarm checkin with no text

commit b4986ba3207374d11438e00d05a6f1ae1720bd49
Author: Jonny Barnes <jonny@jonnybarnes.uk>
Date:   Wed Aug 9 20:57:44 2017 +0100

    A migration that makes the note column of the notes table nullable

commit 80ac4d38c992e59733f60e844f376e36507fb8ee
Author: Jonny Barnes <jonny@jonnybarnes.uk>
Date:   Wed Aug 9 20:57:04 2017 +0100

    Don’t fail when the text content of a note is empty

commit 874acdd3b31028c06d19cdbe9ef34bbc9660a704
Author: Jonny Barnes <jonny@jonnybarnes.uk>
Date:   Wed Aug 9 20:55:12 2017 +0100

    Allow no content for the actual note, maybe its just a picture. Also provide some context for swarm checkins

app/Jobs/SendWebMentions.php

@@ -104,6 +104,9 @@ class SendWebMentions implements ShouldQueue
      */
     public function getLinks($html)
     {
+        if ($html == '' || is_null($html)) {
+            return [];
+        }
         $urls = [];
         $dom = new \DOMDocument();
         $dom->loadHTML($html);