[BUG] Handle Null Intents in onStartCommand

[alkebuware]

Describe the bug I'm getting some Crash reports in Crashlytics where the onStartCommand in IsolateHolderService.kt is called with a null Intent. It seems that variable is currently non-nullable type.

Caused by java.lang.IllegalArgumentException: Parameter specified as non-null is null: method f.n.c.i.d, parameter intent at de.julianassmann.flutter_background.IsolateHolderService.onStartCommand(IsolateHolderService.java:2) at android.app.ActivityThread.handleServiceArgs(ActivityThread.java:4042) at android.app.ActivityThread.-wrap21(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016) at android.os.Handler.dispatchMessage(Handler.java:108) at android.os.Looper.loop(Looper.java:166) at android.app.ActivityThread.main(ActivityThread.java:7425) at java.lang.reflect.Method.invoke(Method.java) at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:245) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:921)

To Reproduce I've actually been unable to reproduce it in my local dev environment.

Expected behavior The onStartCommand should instead take a nullable Intent type as a parameter and the intent.action call should be made null safe

Smartphone:

• Device: Honor 7X, OS: 8.0.0
• Device: LG G6, OS: 9
• Device: A52, OS: 11
• Device: Galaxy Note 8, OS: 9
• Device: Moto X(4), OS: 9
• Device: motorola one 5G UW, OS: 11
• Device: moto z4, OS: 10
• Device: Pixel 2 XL, OS: 11
• Device: Galaxy S8, OS: 9
• Device: Galaxy S21 5G, OS: 11
• Device: Galaxy S7, OS: 8.0.0
• Device: Galaxy S9, OS: 10
• ....

[JulianAssmann]

Should be fixed by #31 Thank you @alkebuware