…ing a `null` `Integer` to an `int` for `batteryCurrentNow` and also do not output `null` values

```
java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.Integer.intValue()' on a null object reference
    at com.termux.api.apis.BatteryStatusAPI$1.writeJson(SourceFile:138)
    at com.termux.api.util.ResultReturner$ResultJsonWriter.writeResult(SourceFile:169)
    at com.termux.api.util.ResultReturner.lambda$returnData$0(SourceFile:271)
    at com.termux.api.util.ResultReturner.$r8$lambda$RFR2zSHu5FsJH7JvuCx4CPnUmMY(SourceFile:0)
    at com.termux.api.util.ResultReturner$$ExternalSyntheticLambda0.run(SourceFile:0)
    at java.lang.Thread.run(Thread.java:1119)
    Suppressed: java.lang.Exception: Called by:
        at com.termux.api.util.ResultReturner.returnData(SourceFile:239)
        at com.termux.api.apis.BatteryStatusAPI.onReceive(SourceFile:27)
        at com.termux.api.TermuxApiReceiver.doWork(SourceFile:91)
        at com.termux.api.TermuxApiReceiver.onReceive(SourceFile:65)
        at android.app.ActivityThread.handleReceiver(ActivityThread.java:4458)
        at android.app.ActivityThread.access$1800(ActivityThread.java:277)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2172)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loopOnce(Looper.java:210)
        at android.os.Looper.loop(Looper.java:299)
        at android.app.ActivityThread.main(ActivityThread.java:8298)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:576)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1073)
```