Remove null fields with `null` value from JSON and YAML output

[jckuester]

This is what it currently looks like:

  {
    "Type": "aws_iam_role",
    "ID": "AWSServiceRoleForAutoScaling",
    "Tags": null,
    "Created": null,
    "Attrs": null
  },

- type: aws_iam_role
  id: AWSServiceRoleForAutoScaling
  tags: {}
  created: null
  attrs: {}