Oh well then that's much easier to solve, I know you said you already solved it, but in your combo box properties, modify the data source to not include Null values or blank values.

example>:
SELECT Conceptual_Entity_ID, Data_Classification, Conceptual_Entity_Name WHERE (((Data_Classification) Not Null) AND Data_Classification <>"")

No need for duplicate tables and you don't see null values in your combo box (provided your query restricts the nulls from the right field).

Glad to help.