I believe that you can use the isnull function which returns a true/false result. You can use this directly in the Access QBE grid definition or within a control on a form.
If I understand what it is that you are trying to do, you'll likely want to use in the query definition.
IsNull Function
Returns a Boolean value that indicates whether an expression contains no valid data (Null).
Syntax
IsNull(expression)
The required expression argument is a Variant containing a numeric expression or string expression.
Remarks
IsNull returns True if expression is Null; otherwise, IsNull returns False. If expression consists of more than one variable, Null in any constituent variable causes True to be returned for the entire expression.
The Null value indicates that the Variant contains no valid data. Null is not the same as Empty, which indicates that a variable has not yet been initialized. It is also not the same as a zero-length string (""), which is sometimes referred to as a null string.
Important Use the IsNull function to determine whether an expression contains a Null value. Expressions that you might expect to evaluate to True under some circumstances, such as If Var = Null and If Var <> Null, are always False. This is because any expression containing a Null is itself Null and, therefore, False.