FlightTaskAuto: fix Weather Vane during landing

Weather vane should only set a yawrate setpoint, but no yaw setpoint.
Setting it to NAN when WV is active makes sure that whatever _yaw_setpoint
is set previously (e.g. through Waypoint) is not used.

Signed-off-by: Silvan Fuhrer <silvan@auterion.com>

src/modules/flight_mode_manager/tasks/Auto/FlightTaskAuto.cpp

@@ -483,7 +483,7 @@ bool FlightTaskAuto::_evaluateTriplets()
 
 	// set heading
 	if (_ext_yaw_handler != nullptr && _ext_yaw_handler->is_active()) {
-		_yaw_setpoint = _yaw;
+		_yaw_setpoint = NAN;
 		// use the yawrate setpoint from WV only if not moving lateral (velocity setpoint below half of _param_mpc_xy_cruise)
 		// otherwise, keep heading constant (as output from WV is not according to wind in this case)
 		bool vehicle_is_moving_lateral = _velocity_setpoint.xy().longerThan(_param_mpc_xy_cruise.get() / 2.0f);