FW Positon Controller: set references to 0 if not provided by local_position (#22101)

* FW Positon Controller: set altitude_ref to 0 if not provided by GPS

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

* FW Positon Controller: set lat/lon reference to 0 if not provided in local_position

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

---------

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

src/modules/fw_pos_control/FixedwingPositionControl.hpp

@@ -266,7 +266,8 @@ private:
 	float _body_velocity_x{0.f};
 
 	MapProjection _global_local_proj_ref{};
-	float _global_local_alt0{NAN};
+
+	float _reference_altitude{NAN}; // [m AMSL] altitude of the local projection reference point
 
 	bool _landed{true};