[BUG][PHP] non-null, negative value cannot be serialized (empty array, false, etc)

Created by: mvehar

Description

non-null, negative value cannot be serialized (empty array, false, etc). problem is IF statement in generated controllers (request?$this->serialize(...)).

openapi-generator version

3.3.4, same problem on 4.0.0

Command line used for generation

PHP symphony templates

Steps to reproduce

Generate pet store, implement listPets with returning empty array, check response -> its empty body, not serialized empty array

Related issues/PRs

/

Suggest a fix/enhancement

Preparing pull request. ($request !== null ? $this->serialize.. )