But it returns an empty result, even if I know there are actors without movies. Conversely, if I try to list actors who have played in at least one movie, the api returns all rows, even those “empty” (https://api.baserow.io/api/database/rows/table/<actors table id>/?user_field_names=true&filter__movies__not_empty).
Try to replace the field name movies in your query with the id of the field which you can find in the API docs: https://api.baserow.io/api/database/rows/table/<actors table id>/?user_field_names=true&filter__field_<id of the field>__empty
Can you check and double check if there are no typo’s in your querystring? My most common error is not combining single underscores for the field id with the double underscores for the filters correctly. So the final result should for example befilter__field_123456__empty
If everything looks fine and it still does not work, I would recommend to create the filter in the grid view and check your browser inspector.
I made another test: I created another row with that field empty. And only this row is correctly filtered. Somehow, the rows I was expected to filter are not filtered. There probably is something in how the data is stored that makes some rows not visible. The bad thing is that I do not remember how I created the original rows.
If needed I can share the original table data (just let me know how, because I see no attachment button).
AndBTW, I must say that what also the web interface has the same behaviour of the API, i.e. when I filter for the empty field the original rows are not visible, but the last I created yesterday is.