AP_Frsky_Telem: allow default arguments in init() to set things to nullptr

libraries/AP_Frsky_Telem/AP_Frsky_Telem.h

@@ -117,7 +117,7 @@ public:
     AP_Frsky_Telem(AP_AHRS &ahrs, const AP_BattMonitor &battery, const RangeFinder &rng);
 
     // init - perform required initialisation
-    void init(const AP_SerialManager &serial_manager, const char *firmware_str, const uint8_t mav_type, AP_Float *fs_batt_voltage, AP_Float *fs_batt_mah, uint32_t *ap_value);
+    void init(const AP_SerialManager &serial_manager, const char *firmware_str, const uint8_t mav_type, AP_Float *fs_batt_voltage = nullptr, AP_Float *fs_batt_mah = nullptr, uint32_t *ap_value = nullptr);
 
     // add statustext message to FrSky lib queue.
     void queue_message(MAV_SEVERITY severity, const char *text);