RFC: Guide for handling generic filter/fieldset errors, such as empty inputs and 'undefined'

This issue is a (choose one):

• Feature request.

Checklist before submitting:

• I've searched for an existing issue.
• The feature I'm asking for is compliant with the JSON:API spec.

Description

A JavaScript client has sent a request https://example.com/v1/books/1337?fields[books]=id,undefined
There are various possible approaches to raise a 400 on the invalid fieldset field undefined. It seems reasonable for JR to recommend some options.

Similarly, a JavaScript client has sent a request https://example.com/v1/books/1337?filter[published-on]=undefined
There are various possible approaches to raise a 400 on the invalid filter value undefined. It seems reasonable for JR to recommend some options.

Lastly, there are a number of cases where blank filter inputs can be considered invalid, such as https://example.com/v1/books/1337?filter[published-on]=&fields[books]=id. There are various possible approaches to raise a 400 on the invalid filter value '(empty)'. It seems reasonable for JR to recommend some options.

No source files or tests are named; start by reviewing the three example requests in the issue and the linked JSON:API specification. Compare the possible handling of undefined and empty field or filter values, then define an agreed recommendation for when the server should return 400 responses.