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

It's common for code to check if a size is null. Add a helper function
to do so.

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

src/libcamera/geometry.cpp

@@ -107,6 +107,12 @@ bool operator==(const Rectangle &lhs, const Rectangle &rhs)
  * \brief The Size height
  */
 
+/**
+ * \fn bool Size::isNull() const
+ * \brief Check if the size is null
+ * \return True if both the width and height are 0, or false otherwise
+ */
+
 /**
  * \brief Assemble and return a string describing the size
  * \return A string describing the size