Nullable object fields don't get generated correctly

[valzam]

Describe the bug
A TS type like this

interface A {
 foo: string;
}

interface B {
 bar: A | null
}

results in the following OpenAPI spec:

components:
 schema:
    B:
      type: object
      properties:
        bar:
          $ref: '#/components/schemas/A'
          nullable: true

This doesn't work as expected in OpenAPI 3.0 (the nullable is simply ignored)
OAI/OpenAPI-Specification#1368

Expected behavior
The field definition needs to use allOf

bar:
  nullable: true
  allOf:
  - $ref: '#/components/schemas/A'

https://stackoverflow.com/questions/40920441/how-to-specify-a-property-can-be-null-or-a-reference-with-swagger

Desktop (please complete the following information):
Spot version 1.9.0

[lfportal]

I do remember this one. To add further context:

spot/lib/src/generators/openapi3/openapi3-specification.ts

Lines 89 to 121 in f93055e

	export interface ReferenceObject {
	$ref: string;
	/**
	* WARNING
	*
	* `nullable: true` will occur when exactly one type reference is combined with null.
	*
	* Example:
	*
	* `MyType | null`
	*
	* https://swagger.io/docs/specification/using-ref/#considerations
	*
	* Schema references cannot contain sibling elements. `nullable` therefore should
	* not be combined with schema reference objects. This rule was misunderstood during
	* development on the OpenAPI 3 generator at Airtasker. This will be removed in a
	* future version of Spot when Airtasker's tooling supports an alternative valid
	* representation for the above scenario.
	*
	* TODO: Find a way to remove this
	* A possible seemingly accepted workaround to this is to wrap the schema reference
	* into an allOf.
	*
	* Example:
	*
	* ```
	* nullable: true
	* allOf:
	* - $ref: #/components/schemas/MyType
	* ```
	*/
	nullable?: boolean;
	}