Conditional column - cannot convert the value null to type text

Hi,

I am trying to add a conditional column that shows "complete" when XYZ columns contain a value (0 included), and "not complete" when any of XYZ columns contains a blank cell (null).

I have added a conditional column as follows:

If [columnX] equals [null] then [not complete]

Else if [columnY] equals [null] then [not complete]
Else if [columnZ] equals [null] then [not complete]

Otherwise ["complete"]

The funny thing is that the conditional columns works as expected in the query editor but when I close&apply the query, it shows the error "cannot convert the value null to type text". How can I check if a cell that contains a number is blank if the value "null" is considered as a text type by the operator?

Any ideas will be greatly appreciated!

Thanks!