ensure we can't have a null in appends

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

@@ -227,8 +227,10 @@ def render("show.json", %{activity: %{data: %{"object" => _object}} = activity}
         |> Enum.filter(fn tag -> is_map(tag) and tag["type"] == "Mention" end)
         |> Enum.map(fn tag -> tag["href"] end)
 
+      to_data = if is_nil(object.data["to"]), do: [], else: object.data["to"]
+
       mentions =
-        (object.data["to"] ++ tag_mentions)
+        (to_data ++ tag_mentions)
         |> Enum.uniq()
         |> Enum.map(fn
           Pleroma.Constants.as_public() -> nil