Add Graph DB Invocations

• SelfDescriptionStoreImpl.java: Add missing calls to graph DB, now that the graph DB interface is available.

• SelfDescriptionStoreImplTest.java, SelfDescriptionControllerTest.java: Update existing tests to store self-descriptions with non-null verification result to avoid exception.

  NOTE: JUnit test for graph DB integration is still in progress and therefore missing from this commit. It will be added later in a separate commit or merge request, as agreed at the meeting yesterday.