There's seemingly no way to do this, likely because of how the data is stored in the database (Date vs plain string) I have a small number of machines that have an empty value in a date based EA and I couldn't figure out a way to gather them. Even using something like more than x days ago and putting in something like 10000 for the value doesn't gather them. : /
I noticed in my case, those few systems with the empty value are not showing anything for it in the API. It just has a <value/> tag where it would normally be <value>[some value]</value>

There is a way to build Smart Groups on empty data in regular string fields, but I was recently told by Jamf this is not recommended. Apparently this puts a strain on the JSS because it's akin to doing a wilcard search against an entire table in the db, versus looking for something specific. Especially if that table is large, like I recently had an issue with, this makes Smart Group calculations that use that information (and use a blank criteria search) eat up a lot of resources and hold onto connections from the Tomcat web apps into the database, so it's not a great idea overall.

Unless this is the way that @mm2270 was told not to use you can do it using two smart groups.

Create one group with the criteria:
XXX More than 0 days
or
XXX Less than 0 days

This will return every computer with a date in the specified field.

Then create a second group with the criteris:
Computer Group Not a member of First Group

This will return all the blank dates for that field.

Thanks for this, @Look - this saves a lot of time when doing a lookup!