Boolean widget with false value not rendered in MD

**Describe the bug**
When setting a Boolean widget to false, this key wont be rendered to the resulting Frontmatter/Markdown file with key: false but will be left out completely. This makes problems with Gatsby GraphQL queries where this field is part of the query.

**To Reproduce**
define boolean field like so:
`- {label: "Featured Post", name: "featuredpost", widget: "boolean", required: false}`

resulting MD file doesnt have the frontmatter key "featuredpost" included.

**Expected behavior**
a file like this gets generated:

```
---
templateKey: single-blog-post
title: My super blog post
featuredpost: false
---
Super Blog content
```

**Applicable Versions:**

- netlify-cms-app 2.14.5
- netlify-cms-core 2.36.4
- Github

**CMS configuration**
not disclosed