The OpenID idToken should never be 'null'

[gbarbon]

Is your feature request related to a problem? Please describe.
The OpenID idToken can never be null. The login method of the GwtAuthorizationServiceImpl class already performs a check. However, if the flow does not pass through that method (e.g. in case of tests), the idToken can be null.

Describe the solution you'd like
Add a check on the login method of the AuthenticationServiceShiroImpl class, and throw an exception if the getIdToken() method returns a null value. The ideal portion of code where the check should be added is here.

Describe alternatives you've considered
N/A

Additional context
The OpenID idToken is returned by the OpenID Provider with the accessToken. We use it in Kapua in order to perform the logout from the OpenID Session on the OpenID Provider side (see here for further information).