As a user I would probably expect to pass a "null" and have the DSL convert that appropriately, or invoke a specific method on the builder to request a match for null.

But assuming that you can check if the requested token is == to the marker token to then consider the validation succesful, then we could explore DSL improvements independently from this issue?

Should the validation also check for the null token and set not enforce a numeric query only against this field?

Sounds like close to what I would do but I'm not sure, so to double-check let me rephrase to see if we're on the same page:
we would still validate that such a query us using exclusively a Numeric type, but also accept the Term which would match with the marker token.