nullable: true don't produce the correct type

[fabien0102]

Describe the bug

On our openAPI generator, nullable: true should produce a | null in the resulted generated type.

Reproduction steps

Schema:

 components:
  schemas:
    PrimitiveStrictValue:
      oneOf:
        - type: string
        - type: boolean
        - type: number

    PrimitiveValue:
      $ref: '#/components/schemas/PrimitiveStrictValue'
      nullable: true

Result:

export type PrimitiveStrictValue = string | boolean | number;

export type PrimitiveValue = PrimitiveStrictValue;

Expected:

export type PrimitiveStrictValue = string | boolean | number;

export type PrimitiveValue = PrimitiveStrictValue | null;

References
https://swagger.io/specification/#schemaNullable

[fabien0102]

OAI/OpenAPI-Specification#1368

This is actually how it's suppose to be (or at least how every other tool treat this problem, so let's be aligned)