Avoid to pass empty array as argument 3 for implementations of ModelManagerInterface::addIdentifiersToQuery()

[phansys]

Subject

Avoid to pass empty array as argument 3 for implementations of ModelManagerInterface::addIdentifiersToQuery().

I am targeting this branch, because this change respects BC.

Closes #6508.

Changelog

### Fixed
- Passing an empty array as argument 3 for implementations of `ModelManagerInterface::addIdentifiersToQuery()`.

[jordisala1991]

Thank you @phansys

[jorrit]

@phansys Why not just return new ArrayCollection() when $value === [] ? The count checking is not needed in this case.

[phansys]

@phansys Why not just return new ArrayCollection() when $value === [] ? The count checking is not needed in this case.

I agree that the count checking is not needed.
I didn't add an early return in order to keep the responsibility of the Collection implementation in TraversableToCollection::transform() using the existing code.
If you think it could be improved, please feel free to open a PR.