Excel 2010, Access 2010, SQL Insert and Update Statments and NULL value in Field

I use Excel to Add new records and to update records in an Access DB. Some Data is required, and some is optional when the record is created; the Update is used to deal with these at a later time. The database connection is: Microsoft.Ace.OLEDB.12.0
If an optional Field is left empty by the User, and I include it in the SQL Insert or Update statement, then I get a run-time error; it does not like data fields that are empty.
How do I include an empty Field among other data-populated Fields in the SQL string? What is the syntax for such a situation.
Right now it is rather complicated because I have to check to see which Fields are empty and exclude them from the SQL statements, but the problem comes back to bite me when a user does an update and the update legitimately is to remove data from a particular Field.
Thanks