How do I validate non-required fields?

[eluser]Skyscraper[/eluser]
@sv3tli0 Again though, there is no form - I am creating an API. Users interact with it via REST.

To give you a use case, imagine an edit-in-place page where a user can update just the Last Name if they want to, without having to update all the rest of the fields. This would be done by sending &last_name=asdf. In this case, first_name is not required. But if they do try to update First Name and set it to empty (&first;_name=), they should get a validation error.