How do I query for empty values?

If you want to query for empty values, please refer to this document.

Ragic currently does not support querying for empty values for the "Select from other sheet" field type. A workaround to query for empty values would work as described below:

Step 1. Create a new free text field

Step 2. Configure formulas so that the field value of new field will equal to the field you would like to query

Step 3. Run a formula recalculation to populate values for your new field.

With this method, you'll be able to query the empty value from your new field.