Don't error if '__session' cookie is missing

[joseph-lozano]

With the current implementation, if the user is not signed in, the application fails with the following error:

[error] ** (FunctionClauseError) no function clause matching in Joken.verify/3
    (joken 2.6.1) lib/joken.ex:247: Joken.verify(nil, nil, [{JokenJwks, [strategy: Clerk.Session.FetchingStrategy]}, Clerk.Session])
    (joken 2.6.1) lib/joken.ex:299: Joken.verify_and_validate/5
    (clerk 1.0.0) lib/authentication_plug.ex:17: Clerk.AuthenticationPlug.call/2

This is because nil is getting passed into Joken.verify/3. Instead, if the __session cookie is missing, we just should respond with a 401 unauthorized, so that the server does not return a 500 response.