Make invariants more robust (for supporting e.g. missing elements, or logical references)

[costateixeira]

Invariants like be-rule-vaccination-4 don't seem to consider for example logical references. Is that intended? Or was it analysed and tested? Suggest confirming and adding some more safeguards, after input from business to confirm the intended behaviour. Ideall for all the invariants.

This is a follow up on #164 which introduces some changes in this sense but may not be complete.

[bdc-ehealth]

@HanneVuegen,
as indicated in mail: https://be-ehealth-standards.atlassian.net/jira/servicedesk/projects/ESI/queues/issue/ESI-878 , this issue will be marked with the next iteration label.