How can I insert a blank field into a date field in Access?

For example, I have a Table with the fields Name, Surname, Birthday(Date).

I'd like to insert a new record based on an excel sheet using ADO & SQL
String. If the birthday field is empty I get an error. The value is ##
because the field is empty.

SQLstring = "INSERT INTO TABLE(NAME, SURNAME, BIRTHDAY) VALUES('Johnny',
'Walker', ##)"

Thanks