Excel PowerQuery: how do I add an IsNotNull column

Question

I have a simple function that I'd like to run on the values in a column, resulting in another column.

    let 
        ThisIsNotNull = (input) => if (input = null) then false else true,
        Source = ...

eventually there is a text column with Nulls in it, let's call it TextColumn. I'd like to add another column alongside it with a value of =ThisIsNotNull(TextColumn).