Align JSON return values on empty response

[yonahd]

Describe the bug
When running kor with --output json we get different responses from resources
e.g.

{
  "default": {
    "ConfigMap": [],
    "Deployment": null,
    "Hpa": null,
    "Ingress": [],
    "Pvc": [],
    "Role": [],
    "Secret": [],
    "Service": null,
    "ServiceAccount": [],
    "Statefulset": null
  }
}

Expected behavior
All resources should return []

OS version, architecture and kor version
e.g. Darwin x86_64
Kor version: 0.1.9

[yonahd]

This is due to the usage of CalculateResourceDifference which will return [] on empty result