BSN validation 0 and 00000000

Added a minor improvement to the validation, but functionally it was good.

The empty function was used to handle usecases when the BSN field is not required. However, in this case it would indeed result in '0' returning a valid BSN number because 0 returns true in empty(). The !isset() does not fix this because it never triggers (it is always set even if empty).

The check needs to be different in case the field is left empty.

Tested a non required empty bsn field which works again with the patch.
0 and 00000000 are still caught as invalid.