Be explicit on logout sending a null value

[nikoskalogridis]

Instead of sending a Maybe value we send explicitly a Encode.null value in the logout function.

The main reason I am proposing this is that SimulatedEffect.Ports.send takes always a Decode.Value so its easier to test this.

It's also not preferred as said in the official elm docs

Sending Json.Decode.Value through ports is recommended, but not the only way