Delete the value of an optional text/image widget results in an empty string value

[hangvane]

Describe the bug

If a text/image widget is required: false, removing the existing value of the widget would left an empty string value, rather than undefined or null.

To Reproduce

1. Define an optional text/image widget

    fields: # The fields for each document, usually in front matter
      - {label: 'Excerpt', name: 'excerpt', widget: 'text', required: false}
      - {label: 'Image', name: 'photos', widget: 'image', required: false}

2. Create a post with the widgets edited
3. Edit the post again to delete the value of the widgets
4. The field of two widgets left in the generated markdown file with empty strings:

excerpt: ""
photos: ""

Expected behavior

Remove the fields completely if the widget is optional and empty.

Screenshots

Applicable Versions:

• Decap CMS version: 3.1.1

CMS configuration

Additional context

This bug is issued in #995, #2007, #2848, #1449, #2017. As replied #995 (comment), seems this bug should be fixed since 3.0.0.