I have natural stone veneer on the bottom half of my exterior wall, transitioning to hardie plank on top half of wall.  No rainscreen was installed, just osb sheathing, then tyvek housewrap, then siding. No gap behind the hardie for drainage, though there is behind the stone.   I have seen that through wall flashing is recommended at the transition between siding materials.  We have an opportunity to add that right now, but does it make sense if we have no rainscreen system?