Reject resolve calls returning nil

[vemv]

Brief

Fixes #9

QA plan

Have a look at the new tests

Author checklist

• I have QAed the functionality
• The PR has a reasonably reviewable size and a meaningful commit history
• I have run the branch formatter and observed no new/significative warnings
• I have self-reviewed the PR prior to assignment, and its build passes
• Specifically, I have code-reviewed iteratively the PR considering the following aspects in isolation:
  • Correctness
  • Robustness (red paths, failure handling etc)
  • Modular design
  • Test coverage
  • Spec coverage
  • Documentation
  • Security
  • Performance

Reviewer checklist

• I have checked out this branch and reviewed it locally, running it
• I have QAed the functionality
• I have code-reviewed iteratively the PR considering the following aspects in isolation:
  • Business-facing correctness
  • Robustness (red paths, failure handling etc)
  • Modular design
  • Test coverage
  • Spec coverage
  • Documentation
  • Security
  • Performance

[vemv]

The tests (old and new) are not very fine-grained, would be happy to improve them after https://github.com/nedap/utils.spec/pull/55

[thumbnail]

The tests (old and new) are not very fine-grained, would be happy to improve them after nedap/utils.spec#55

Let's do that in a different pr