Fix/4372 skip null values in hal

[BacLuc]

Q	A
Branch?	main
Tickets	Issue: #4372
License	MIT
Doc PR	-

RFC: This PR adds support for skip_null_to_one_relations: false with relations to the HAL normalizer. This however deviates from the hyperschema, because now the entries in _links and _embedded may be null, which isn't really allowed in HAL.

Relates to:

• fix: Respect skip_null_values in the HAL normalizer #4373

Additionally:

• Always add tests and ensure they pass. ✔️
• Never break backward compatibility (see https://symfony.com/bc). ✔️
• Update CHANGELOG.md file: Will do that if this has a chance of beeing merged.

[soyuka]

This looks okay, could you fix the tests? (lowest branches tests don't matter they'll get fixed later)

[BacLuc]

This looks okay, could you fix the tests? (lowest branches tests don't matter they'll get fixed later)

I fixed most of them.
It didn't work with mongodb, is it ok if i leave it skipped or can you give me a hint were i should look?

I also did not change the changelog, because i don't have a hash of a commit in the api-platform repository yet.
Is that ok?

[soyuka]

Thanks!