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.