You can use the filtering arrow buttons at the right of each field name in the listing page. Check the Exact match checkbox and click Submit for Ragic to return entries that have the filtered field empty.
You can also use the left sidebar search tool to query for empty values with the same method.
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.