Describe the bug
It's currently possible to inject front matter properties within the body field.
This means CMS users can:

• Introduce new front matter properties that were never planned;
• Potentially clash with other reserved properties such as layout or slug;
• Mistakenly introduce properties with values as null;

I also tried overriding a property that as filled in above and the field seems to take precedence here, which is great. So at least I hope this prevents validation bypassing?

To Reproduce

1. Have CMS collection expecting an optional state field along with some other mandatory fields including a body
2. Create a new record there and leave state field empty
3. In the body field, add state: in the front matter
4. Check the generated file with state: null

Expected behavior
Ignore front matter altogether within the body. That's not what it's meant for.

Applicable Versions:

• Netlify CMS version: 2.14.20
• Git provider: github
• OS: macOS 11.0.1 (20B29)
• Browser version: Brave v1.18.70
• Node.JS version: v12.17.0

CMS configuration

  - name: 'docs'
    label: 'Docs'
    folder: '_pages/docs'
    create: true
    fields:
      - name: 'title'
        label: 'Title'
        widget: 'string'

      - name: 'state'
        label: 'State'
        widget: 'string'
        required: false

      - name: 'body'
        label: 'Body'
        widget: 'markdown'