Update example app on Flutter plugin (#220)

* Update example app on Flutter plugin

* Expose `empty_wallet_cache` through Dart bindings (#224)

packages/flutter/example/lib/routes/home/widgets/balance.dart

@@ -0,0 +1,36 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_breez_liquid/flutter_breez_liquid.dart';
+
+class Balance extends StatelessWidget {
+  final Stream<GetInfoResponse> walletInfoStream;
+
+  const Balance({super.key, required this.walletInfoStream});
+
+  @override
+  Widget build(BuildContext context) {
+    return StreamBuilder<GetInfoResponse>(
+      stream: walletInfoStream,
+      builder: (context, walletInfoSnapshot) {
+        if (walletInfoSnapshot.hasError) {
+          return Center(child: Text('Error: ${walletInfoSnapshot.error}'));
+        }
+
+        if (!walletInfoSnapshot.hasData) {
+          return const Center(child: Text('Loading...'));
+        }
+
+        if (walletInfoSnapshot.requireData.balanceSat.isNaN) {
+          return const Center(child: Text('No balance.'));
+        }
+        final walletInfo = walletInfoSnapshot.data!;
+
+        return Center(
+          child: Text(
+            "${walletInfo.balanceSat} sats",
+            style: Theme.of(context).textTheme.headlineLarge?.copyWith(color: Colors.blue),
+          ),
+        );
+      },
+    );
+  }
+}