bool = csurface:IsNull()
True if the surface is NULL, i.e. invalid. If functions
that allocate surfaces fail, they might not throw an error but simply set the
surface to NULL. You can use this function to check if surface allocation has
failed in which case the surface will be NULL.
Also, certain getter functions can return a NULL object in case the object
doesn't exist. You can use this function to check if a getter function returned
a NULL handle.
True if the surface is NULL, otherwise False