Image widget default value isn't being preserved

[smashercosmo]

Describe the bug
According to image widgets docs, widget's default value is null. But when I try to save a record without choosing any image, it just omits the property completely.

To Reproduce

1. Try to create a new record without selecting any image
2. Check that newly created json won't have image field at all

Expected behavior
If image hasn't been provided, image property should have null as a value

Applicable Versions:

  System:
    OS: macOS 10.15.7
  Binaries:
    Node: 14.17.1 - /usr/local/opt/node@14/bin/node
    npm: 7.19.0 - /usr/local/bin/npm
  npmPackages:
    netlify-cms-app => 2.15.20

CMS configuration

backend:
  name: git-gateway
media_folder: /static/images
public_folder: /images
collections:
  - label: Pictures
    name: pictures
    folder: content/pictures
    format: json
    create: true
    media_folder: '{{media_folder}}/pictures'
    public_folder: '{{public_folder}}/pictures'
    fields:
      - { label: Title, name: title, widget: string }
      - { label: Image, name: image, widget: image, required: false }

P.S. If it's not something obvious or expected behaviour, I can create minimal example to demonstrate the issue

[smashercosmo]

I've just realised that it's not a problem for my personal use case, so if you think, that's it's not really an issue, feel free to close