FlightTaskAuto: accept invalid triplet_setpoint (and interpret as loiter)

This will allow navigator to not output any setpoints while disarmed.
Otherwise the position controller outputs warnings in the form of:
Auto activation failed with error: Activation Failed

The risk here is that it could hide problems, where navigator sends invalid
triplets when it should not.

src/lib/flight_tasks/tasks/Auto/FlightTaskAuto.cpp

@@ -137,10 +137,13 @@ bool FlightTaskAuto::_evaluateTriplets()
 	// Check if triplet is valid. There must be at least a valid altitude.
 
 	if (!_sub_triplet_setpoint.get().current.valid || !PX4_ISFINITE(_sub_triplet_setpoint.get().current.alt)) {
-		// Best we can do is to just set all waypoints to current state and return false.
+		// Best we can do is to just set all waypoints to current state
 		_prev_prev_wp = _triplet_prev_wp = _triplet_target = _triplet_next_wp = _position;
-		_type = WaypointType::position;
-		return false;
+		_type = WaypointType::loiter;
+		_yaw_setpoint = _yaw;
+		_yawspeed_setpoint = NAN;
+		_updateInternalWaypoints();
+		return true;
 	}
 
 	_type = (WaypointType)_sub_triplet_setpoint.get().current.type;