Set clear value as null

[dikwickley]

Fixes #3308

When removing a Linked Record, it is set to undefined. This PR updates that to null.
Well, it was a one-line change 😅. Explanation below.

Technical details

So the issue was the value for the foreign key when deleted was set as undefined when deleted from the input.

But this undefined value is skipped in the request payload

By setting the clear value as null, it is sent in the request payload and hence record removal is reflected after saving.

@seancolsen can you please take a look at this?

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[seancolsen]

This works great. Thanks @dikwickley!