$metadata->thumbnail_url is not guaranteed to be set

We should probably have a separate class for this, so we can more
easily combine different technologies similar to oEmbed/OpenGraph.

plugins/Oembed/OembedPlugin.php

@@ -73,6 +73,7 @@ class OembedPlugin extends Plugin
             $metadata = OpenGraphHelper::ogFromHtml($dom);
         }
 
+        if (isset($metadata->thumbnail_url)) {
             // sometimes sites serve the path, not the full URL, for images
             // let's "be liberal in what you accept from others"!
             // add protocol and host if the thumbnail_url starts with /
@@ -86,6 +87,7 @@ class OembedPlugin extends Plugin
             if($metadata->thumbnail_url == 'https://s0.wp.com/i/blank.jpg') {
                 unset($metadata->thumbnail_url);
             }
+        }
 
     }