is it possible to set a null field default value?

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I would like to process some requests to a service differently if one of the requesting interface fields is empty but as far as I can tell it does not seem to be possible. Sending the request without setting the field results in the field default value e.g. 'false' being sent which makes enough sense but there does not seem to be a way to set the field default value to anything indicating it not being initialized. I tried setting to null or none but building says it must be true or false