Im using a CQWP and need to display all items that have a column that is empty. The column is a lookup but I've tried a normal (single line of text) column but cqwp shows no filtering for null or empty values.
To further Tomislav's answer, set the calculated column to return Yes/No:
Then filter the CQWP with Display = 1
Edit: This doesn't work because apparently the generated CAML Query always sets the field type to text for calculated columns, according to this: https://github.com/pnp/sp-dev-fx-webparts/issues/758