[BUG] codegen: uncompilable validators for nullable field types

[hughsimpson]

    DummyClass:
      title: DummyClass
      type: object
      description: I model something
      properties:
        name:
          type: string
          minLength: 1
          nullable: true
          description: I am a name

generates validators that don't compile. Removing nullable: true would produce something that compiles. Validators for nullable declarations on field types handle the optionality 'inside' themselves, whereas validators for the 'non-requiredness' of a field handle the optionality before passing through. Need to dedup that here.

Noting here so I don't forget