Skip reporter queries with empty states or queues

Querying with an empty list may cause the following error:

```
(Postgrex.Error) ERROR 42809 (wrong_object_type) op ANY/ALL (array)
```

This fixes the erorr by skipping queries for empty lists. This could
happen if all of the states were above the estimate threshold, or no
active queues were found.

Closes oban-bg/oban#1182

lib/oban/met/reporter.ex

@@ -182,14 +182,19 @@ defmodule Oban.Met.Reporter do
     end)
   end
 
-  defp count_query(states) do
+  defp count_query([]), do: where(Job, [_], false)
+
+  defp count_query(states) when is_list(states) do
     Job
     |> select([j], %{series: :full_count, state: j.state, queue: j.queue, value: count(j.id)})
     |> where([j], j.state in ^states)
     |> group_by([j], [j.state, j.queue])
   end
 
-  defp guess_query(states, queues, conf) do
+  defp guess_query([], _queues, _conf), do: where(Job, [_], false)
+  defp guess_query(_states, [], _conf), do: where(Job, [_], false)
+
+  defp guess_query(states, queues, conf) when is_list(states) and is_list(queues) do
     from(p in fragment("json_array_elements_text(?)", ^queues),
       cross_join: x in fragment("json_array_elements_text(?)", ^states),
       select: %{