libcamera: geometry: Add isNull() function to Rectangle class

It's common for code to check if a rectangle is null. Add a helper function
to do so and test the function in test/geometry.cpp

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>

include/libcamera/geometry.h

@@ -181,6 +181,7 @@ public:
 	unsigned int width;
 	unsigned int height;
 
+	bool isNull() const { return !width && !height; }
 	const std::string toString() const;
 };