refactor: change confirmations to non-late and add default value (#3148)

cw_core/lib/transaction_info.dart

@@ -10,7 +10,7 @@ abstract class TransactionInfo extends Object with Keyable {
   late bool isPending;
   late DateTime date;
   int? height;
-  late int confirmations;
+  int confirmations = 0;
   String amountFormatted();
   String fiatAmount();
   String? feeFormatted();