Interface doesn't support nullable boolean

[paulboudewijn]

Preflight Checklist

• I have completed all Troubleshooting Steps.
• I'm on the latest version of Directus.
• There's no other issue that already describes my problem.

Describe the Bug

Nullable boolean fields can't be set to null again after they have been set to true or false through the boolean interface.
Allthough I'm fairly new to Directus, NodeJS and VueJS, I have a fix ready to solve this.

To Reproduce

1. add a nullable boolean field to a collection
2. add an item to the collection with the boolean set to null (this works)
3. notice that in the collection list the boolean value is set to 'false' (not supposed to happen)
4. check in the database to see the value of this boolean is in fact still 'null'
5. edit this collection item; notice the checkbox is set to 'false' not 'null' (not supposed to happen)
6. click to checkbox to set the value to 'true'. save the changes.
7. edit the item again and notice the value can't be set to 'null' again. Only 'true' or 'false' are available. (not supposed to happen)

What version of Directus are you using?

v9.0.0.rc94

What version of Node.js are you using?

v16.10.0

What database are you using?

MS SQL

What browser are you using?

Chrome

What operating system are you using?

Windows 10

How are you deploying Directus?

running locally