Refactor percentage clauses

Author: zoldar

extra/lib/plausible/stats/funnel.ex

@@ -168,11 +168,7 @@ defmodule Plausible.Stats.Funnel do
     |> Enum.reverse()
   end
 
-  defp percentage(x, y) when x in [0, nil] or y in [0, nil] do
-    "0"
-  end
-
-  defp percentage(x, y) do
+  defp percentage(x, y) when is_integer(x) and x > 0 and is_integer(y) and y > 0 do
     result =
       x
       |> Decimal.div(y)
@@ -187,4 +183,8 @@ defmodule Plausible.Stats.Funnel do
       decimal -> decimal
     end
   end
+
+  defp percentage(_x, _y) do
+    "0"
+  end
 end