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.
Workaround - create new Calculated column "Display".
Formula should be: =IF(LEN(LookupCol)=0;"Yes";"No").
Eventually, make a filter based on Display column: Show items when Display is equal to Yes.