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>

src/libcamera/geometry.cpp

@@ -385,6 +385,12 @@ bool operator==(const SizeRange &lhs, const SizeRange &rhs)
  * \brief The distance between the top and bottom sides
  */
 
+/**
+ * \fn bool Rectangle::isNull() const
+ * \brief Check if the rectangle is null
+ * \return True if both the width and height are 0, or false otherwise
+ */
+
 /**
  * \brief Assemble and return a string describing the rectangle
  * \return A string describing the Rectangle