migrated data_util.dart to null safety

lib/ui/payment/subscription_common_widgets.dart

@@ -87,7 +87,7 @@ class ValidityWidget extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     if (currentSubscription == null) {
-      return Container();
+      return const SizedBox.shrink();
     }
     final endDate = getDateAndMonthAndYear(
       DateTime.fromMicrosecondsSinceEpoch(currentSubscription.expiryTime),

lib/utils/data_util.dart

@@ -1,5 +1,3 @@
-// @dart=2.9
-
 import 'dart:math';
 
 double convertBytesToGBs(final int bytes, {int precision = 2}) {