I don`t know what type of error you had with your boolean filter but here is calculation returning 1 if the measure related by the parameter is not null.
By filtering at 1 you would get rid of the nulls.
case [Select Score]
when 'GAD7SCORE' then if isnull([GAD7SCORE]) then 0 else 1 end
when 'PHQ9_SCORE' then if isnull([PHQ9_SCORE]) then 0 else 1 end
Thanks v much for your quick response. That does just the thing.