fix: remove false-positives in validation

Don't assign any artificial value to NIP field if user has not passed any value themself. Such behavior produces false-positives in validation with FCF plugin, when non-required field prevents checkout from completion.

Possibly, this value should be checked somehow (especially in case user doesn't pass any value when we do require this value), but such check should be implemented in more sensitive approach than just assigning invalid value.

Signed-off-by: Bart Jaskulski bjaskulski@protonmail.com