Update battery status output on missing battery

Config.h

@@ -169,6 +169,7 @@
 	const uint8_t RX_ONGOING = 0x04;
 
     // Power management
+    #define BATTERY_STATE_UNKNOWN     0x00
     #define BATTERY_STATE_DISCHARGING 0x01
     #define BATTERY_STATE_CHARGING    0x02
     #define BATTERY_STATE_CHARGED     0x03

Power.h

@@ -190,7 +190,7 @@ void measure_battery() {
           }
         }
       } else {
-        battery_state = BATTERY_STATE_DISCHARGING;
+        battery_state = BATTERY_STATE_UNKNOWN;
         battery_percent = 0.0;
         battery_voltage = 0.0;
       }