NULL is "unknown" value. Pretty much any operation that you perform on NULL returns NULL. So, if you try to ask "is this equal to something whose value you don't know", it can only answer "don't know".
There is one other way to do what you're interested in doing, a technique called a "substitution conversion".
Oconv(arg1,"S;x;y;z") returns x if the value of arg1 (in a Boolean context) is "true", y if the value of arg1 is "false", and z if the value of arg1 is NULL. An asterisk means "leave the value of arg1 unchanged".
In your example, you could use