schema-tree: Null marker to array-of-arrays/objects/primitives/union

[HansAarneLiblik]

Add null|markers to array properties. Also updates previously added null| markers for object properties

Fixes #952

For schema

Pet:
  type: object
  properties:
    primitive_array:
      type: array
      nullable: true
      items:
        type: string
    object_array:
      type: array
      nullable: true
      items:
        $ref: '#/components/schemas/Category'
    array_array_primitive:
      type: array
      nullable: true
      items:
        type: array
        items:
          type: string
    array_array_object:
      type: array
      nullable: true
      items:
        type: array
        items:
          $ref: '#/components/schemas/Category'
    anyof_array:
      type: array
      nullable: true
      items:
        anyOf:
          - type: string
          - $ref: '#/components/schemas/Category'

Before:

After:

[mrin9]

Thanks for the contribution !!!
But we have just restarted our work on this project and we are in a mid of some refactoring. I will take a look at your code. But you may have to fix the conflicts again. or i will extract the core logic and incorporate it once we are done with our refactoring cycle.