ekf2: only populate gnss pos aid src status if ref initialized

- this is a minor logging improvement when plotting the position from the beginning of the log (often a replay session)

src/modules/ekf2/EKF/gps_control.cpp

@@ -80,8 +80,11 @@ void Ekf::controlGpsFusion(const imuSample &imu_delayed)
 			}
 		}
 
+		if (_pos_ref.isInitialized()) {
+			updateGnssPos(gnss_sample, _aid_src_gnss_pos);
+		}
+
 		updateGnssVel(gnss_sample, _aid_src_gnss_vel);
-		updateGnssPos(gnss_sample, _aid_src_gnss_pos);
 
 	} else if (_control_status.flags.gps) {
 		if (!isNewestSampleRecent(_time_last_gps_buffer_push, _params.reset_timeout_max)) {