mastodon api: fix rendering of cards without image URLs (closes #597)

lib/pleroma/web/mastodon_api/views/status_view.ex

@@ -192,8 +192,12 @@ def render("card.json", %{rich_media: rich_media, page_url: page_url}) do
     page_url = page_url_data |> to_string
 
     image_url =
+      if rich_media[:image] != nil do
         URI.merge(page_url_data, URI.parse(rich_media[:image]))
         |> to_string
+      else
+        nil
+      end
 
     site_name = rich_media[:site_name] || page_url_data.host