Fixes lateinit property has not been initialized crash

[dhmaciel]

This MR prevents the crash where, in some cases, the lateinit variable is null when onActivityResult is called. To prevent it, it was changed from lateinit to a nullable variable, where the var is checked before using it.

Caused by s9.o: lateinit property loadPaymentDataResult has not been initialized at io.flutter.plugins.pay_android.GooglePayHandler.onActivityResult(GooglePayHandler.java:4) at io.flutter.embedding.engine.FlutterEngineConnectionRegistry$FlutterEngineActivityPluginBinding.onActivityResult(FlutterEngineConnectionRegistry.java:25) at io.flutter.embedding.engine.FlutterEngineConnectionRegistry.onActivityResult(FlutterEngineConnectionRegistry.java:13) at io.flutter.embedding.android.FlutterActivityAndFragmentDelegate.m(FlutterActivityAndFragmentDelegate.java:16) at io.flutter.embedding.android.FlutterFragment.onActivityResult(FlutterFragment.java:10) at io.flutter.embedding.android.FlutterFragmentActivity.onActivityResult(FlutterFragmentActivity.java:5) at android.app.Activity.dispatchActivityResult(Activity.java:9362) at android.app.ActivityThread.deliverResults(ActivityThread.java:6055) at android.app.ActivityThread.handleSendResult(ActivityThread.java:6101) at android.app.servertransaction.ActivityResultItem.execute(ActivityResultItem.java:67) at android.app.servertransaction.ActivityTransactionItem.execute(ActivityTransactionItem.java:45) at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:139) at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:96) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2685) at android.os.Handler.dispatchMessage(Handler.java:106) at android.os.Looper.loopOnce(Looper.java:230) at android.os.Looper.loop(Looper.java:319) at android.app.ActivityThread.main(ActivityThread.java:8918) at java.lang.reflect.Method.invoke(Method.java) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:608) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1103)

To prevent the crash when lateinit is null, I changed the variable to nullable and checked it every time that is used, if for some reason the onActivityResult method is called, the app will not break.