PUT orders - handle empty array of the PO lines

Requirement:
In case a request that contains an empty po_lines array, all the PO Lines should be removed... e.g. per the "if the po_line exists in the database but not in the request, remove the po_line" rule.

Current situation:
The resulting CompositePurchaseOrder.java model generated based on composite_purchase_order.json schema has

    @JsonProperty("po_lines")
    @JsonPropertyDescription("a list of completely de-referenced purchase order lines")
    @Valid
    private List<PoLine> poLines = new ArrayList<PoLine>();

This means that if request does not include po_lines or includes "po_lines": [] the result is the same i.e. empty List.

Required change:
The schema change is required to make these 2 cases different:

...
    "po_lines": {
      "description": "a list of completely de-referenced purchase order lines",
      "id": "po_lines",
      "type": "array",
      "items": {
        "type": "object",
        "$ref": "composite_po_line.json"
      },
      "default": null
    },
...

In this case if the request does not include po_lines or includes "po_lines": null the result will be absolutely the same i.e. "do not touch PO lines". The business logic should start processing the PO lines only if the field is not null.