Test of $_POST Variable's Existance Fails if Value=0

I always believed that if I had a test like this in a server action:

Then the condition will be false if the $_POST variable is null, and true in all other cases.

However this example I am working with also returns false if the $_POST variable has the value of 0.

Did I misunderstand how this works, or is it a bug? I am still on 3.9.3.

How can I structure the test so that it will return false ONLY when the $_POST variable is null?