Refines a filter to match items whose specified property does not exist or does not have any value.
The isEmpty() function refines a WixDataFilter to only match items where the
value of the specified property is null or undefined or the property does
not exist.
If the property contains any value at all for a given item, including the empty string or an invalid value, that item will match the query.
The the property in which to check for a value.