Avoid to call registrationStore.removeObservation with null registration ID

[sbernard31]

This aims to fix #1384

But working on this, I also polish some details which sounds not so good to me :

• NPE in RedisRegistrationStoreTest
• lastEndpointUsed in Registration should be mandatory
• add validation on lastEndpointUsed
• Remove redundant validation in Registration Constructor
• Avoid any Registration call with null value for registration ID.

[cyril2maq]

I did run my tests which led to create this issue with this PR, it is now OK👍

Regarding the last commit, I think an additional test could be interesting. Maybe a unit test on LwM2mObservationStore.remove(Token token) method checking that the observation has been removed.

Suggestion for LwM2mObservationStoreTest :

    @Test
    public void remove_observation() {
        // given
        givenASimpleRegistration(lifetime);
        store.addRegistration(registration);

        org.eclipse.californium.core.observe.Observation observationToStore = prepareCoapCompositeObservation();
        observationStore.put(exampleToken, observationToStore);

        // when
        observationStore.remove(exampleToken);

        // then
        Observation leshanObservation = store.getObservation(registrationId,
                new ObservationIdentifier(exampleToken.getBytes()));
        assertNull(leshanObservation);
    }

[sbernard31]

I added your suggestions (commit dbe5aa2)

I guess I can merge the PR in master now ?

[cyril2maq]

I guess I can merge the PR in master now ?

Yes !

[sbernard31]

Thx for reviewing this 😉