Ok I thought we were talking about conditional expressions.
I guess it depends on exactly what kind of expression 'null in (x, y)' should be. A string? expression or a conditional expression.
Conditional expressions could IMO safely be replaced with what was already discussed.
Another question is if it should even be allowed to use this expression as string? expression. If you want to generate null, you can do so with case when where the null expression acts as conditional expression again.