forked from loafle/openapi-generator-original
* Add required enum property
* Update samples
* Add test case which reproduce the problem
refs https://github.com/swagger-api/swagger-codegen/pull/7686#issuecomment-368200011
> 2. Non-required enum property is listed as invalid when omitted
* If the property is not empty, perform validation
* Update samples
* Use is_null() according to setter implementation
refs 377247f125/modules/swagger-codegen/src/main/resources/php/model_generic.mustache (L347)
* Update samples