Encode non-present battery as -1

meshtastic/telemetry.proto

@@ -13,9 +13,9 @@ option swift_prefix = "";
  */
 message DeviceMetrics {
   /*
-   * 0-100 (>100 means powered)
+   * 0-100 (>100 means powered, -1 means not connected)
    */
-  optional uint32 battery_level = 1;
+  optional int32 battery_level = 1;
 
   /*
    * Voltage measured