I know you decided on a "solution" but I disagree.
It would make more sense to make the date field a variant so you could check for null. That at least would correspond with how you would work with a control or a date field in a query which don't have defaults.
Since the variant is a non-specific data type, it won't restrict the input so to check for a valid date, you would use the IsDate() function. And keep in mind, just because Access thinks something is a valid date doesn't mean the data is rational in your context. For example #01/01/202# is a valid date as far as Access is concerned but it is a typo and not rational in context.