Fix characteristic warning (#1104)

## ♻️ Current situation

This warning can appear:
```
This plugin generated a warning from the characteristic 'Battery Level': characteristic value expected valid finite number and received "undefined" (undefined). See https://homebridge.io/w/JtMGR for more info.
```

See #1093.

src/device/curtain.ts

@@ -392,13 +392,15 @@ export class Curtain extends deviceBase {
       this.debugLog(`LightLevel: ${this.serviceData.lightLevel}, CurrentAmbientLightLevel: ${this.LightSensor.CurrentAmbientLightLevel}`)
     }
     // BatteryLevel
-    this.Battery.BatteryLevel = this.serviceData.battery
-    this.debugLog(`BatteryLevel: ${this.Battery.BatteryLevel}`)
-    // StatusLowBattery
-    this.Battery.StatusLowBattery = this.Battery.BatteryLevel < 10
-      ? this.hap.Characteristic.StatusLowBattery.BATTERY_LEVEL_LOW
-      : this.hap.Characteristic.StatusLowBattery.BATTERY_LEVEL_NORMAL
-    this.debugLog(`StatusLowBattery: ${this.Battery.StatusLowBattery}`)
+    if (this.serviceData.battery) {
+      this.Battery.BatteryLevel = this.serviceData.battery
+      this.debugLog(`BatteryLevel: ${this.Battery.BatteryLevel}`)
+      // StatusLowBattery
+      this.Battery.StatusLowBattery = this.Battery.BatteryLevel < 10
+        ? this.hap.Characteristic.StatusLowBattery.BATTERY_LEVEL_LOW
+        : this.hap.Characteristic.StatusLowBattery.BATTERY_LEVEL_NORMAL
+      this.debugLog(`StatusLowBattery: ${this.Battery.StatusLowBattery}`)
+    }
   }
 
   async openAPIparseStatus(): Promise<void> {