You can only test if a variable is null if that variable is an Object type variable. You can't assign null values to primitive data types, so you can't test them either.

From your code, I don't know if 'x' is a primitive data type or not, but I assumed it was.

Null values are just a special value to indicate that a variable that normally would hold an object, doesn't hold anything.

Example:
Code:

PImage img;
void setup() {
    img = null;
}

void draw() {
    if (img != null) {
	  image(img, 0, 0);
    }
}
void keyPressed() {
    img = loadImage("image.jpg");
}
 

In the example, null is assigned to 'img' to indicate that it wasn't initialized yet (so it doesn't have a reference to a valid image for us to draw).