[PHP] Handle non-required property as nullable

Description

When a property isn't required, it is not shown in Doc Block, that the getter can actually return "null".
Also it is not shown in the setter, that "null" is a valid value

Swagger-codegen version

2.3

Swagger declaration file content or url

  /path
    post:
      parameters:
        - in: body
          required: true
          name: objectName
          schema:
            type: object
            required:
              - requiredPropertyName
            properties:
              requiredPropertyName:
                type: string
              notRequiredPropertyName:
                type: string

Suggest a fix/enhancement

Change returnType in Doc Block for getters to correctly show, that the returned value can be null (if the property is not required).
Also shown in Doc Block for setters, that value can be null (if property is not required)

Start by locating the PHP generation templates or entry points that produce model getter and setter DocBlocks, then reproduce the provided schema with one required and one non-required property. Done means the generated documentation marks the optional property's getter return value and setter argument as nullable, while the required property remains non-nullable.