null values are passed in to the db2
What are you calling "null values" here? Every character on a computer has a value -- from X'00' to X'FF' on a mainframe. If you are referring to X'00', that is NOT a "null value" but actually 8 bits all set to zero -- in COBOL this is called LOW-VALUE as opposed to all 8 bits being set to 1, X'FF', which is HIGH-VALUE in COBOL.
In DB2, a NULL is a specific term used to denote the absence of a value, and the only way to set one up is to use a flag byte that indicates that the variable either has a value or it is NULL.