Fix regression of nullability on allOf with $ref

Created by: stjernegard

I believe this fixes #10780, though there's some differences in our examples, so I can't be entirely sure. I can reproduce this regression between 5.2.1 and 5.3.0, so it probably is.

There's possibly a deeper problem here, as I'm not sure if these composed types are supposed to be handled by updatePropertyForAnyType(). But since they currently are, I kept my changes to a minimum. I found that ModelUtils.isNullable() returns the correct value here, as it recognises that composed types are only nullable in some cases and not all cases, as updatePropertyForAnyType() otherwise suggests.

An object with the definition below would currently be generated with a nullable value3 property. I believe that's a bug, as that is not the default behaviour with either value1 or value2 in my example.

---
openapi: 3.0.3
info:
  title: Foo
  version: v1
servers:
- url: /
  description: Foo
paths:
  /foo:
    get:
      responses:
        "200":
          description: OK
components:
  schemas:
    Bar:
      required:
        - value1
        - value2
        - value3
      type: object
      properties:
        value1:
          type: string
        value2:
          allOf:
            - type: string
        value3:
          allOf:
            - $ref: '#/components/schemas/Foo'
    Foo:
      type: string

(Swift is my language of choice for the example, but this change is not specific to any language, as the change is in DefaultCodegen.java)

Before my change:

[...]
public struct Bar: Codable, JSONEncodable, Hashable {

    public var value1: String
    public var value2: String
    public var value3: String?
[...]

After my change:

[...]
public struct Bar: Codable, JSONEncodable, Hashable {

    public var value1: String
    public var value2: String
    public var value3: String
[...]

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Since this is not targeting a particular language, I hope it's okay I ping the core team instead. cc @wing328 @jimschubert @cbornet @jmini @etherealjoy @spacether