[zfcampus/zf-rest#31] Allow null values for entity identifiers

- Actually, `ZF\Hal\Entity` allowed it previously; the new test passed
  even without changes. Simply modified the code to explicitly make the
  `$id` an optional argument.
[zfcampus/zf-rest#31] Handle null ids for entities

- Do not return an ApiProblem in the case that an entity's identifier
  was not discovered.
- Updated `injectSelfLink()` to not include null identifiers for
  entities.
[#39] Updated group annotations for new tests

- Forgot that we had an issue in zf-hal for this
Merge branch 'hotfix/zf-rest-31'

Close #39