Fix another dialyzer regression

Author: zoldar

lib/plausible.ex

@@ -31,11 +31,7 @@ defmodule Plausible do
           unquote(term)
 
         1 ->
-          case unquote(term) do
-            value when is_boolean(value) -> not value
-            nil -> :something
-            _other -> nil
-          end
+          not unquote(term)
       end
     end
   end

lib/plausible/teams/invitations.ex

@@ -543,7 +543,11 @@ defmodule Plausible.Teams.Invitations do
     end
   else
     def ensure_can_take_ownership(_site, _team, _opts) do
-      always(:ok)
+      if always(true) do
+        :ok
+      else
+        {:error, :no_plan}
+      end
     end
   end