Any field that is part of a key is never stored as null. (A null key does not have a well-defined position in relation to other keys.) If a key field has an illegal value, the field is stored in the database as described here:
Illegal LOW-VALUES: minimum possible value (0 or -99999...)
Illegal HIGH-VALUES: maximum possible value (99999...)
Illegal SPACES: zero (or 01/01/01 for a date field)