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

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

@@ -192,8 +192,12 @@ defmodule Pleroma.Web.MastodonAPI.StatusView do
     page_url = page_url_data |> to_string
 
     image_url =
-      URI.merge(page_url_data, URI.parse(rich_media[:image]))
-      |> to_string
+      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