[flutter_tools] re-enable non-nullable test (#57075)

dev/bots/test.dart

@@ -604,7 +604,6 @@ Future<void> _runFrameworkTests() async {
     await _runFlutterTest(path.join(flutterRoot, 'packages', 'fuchsia_remote_debug_protocol'), tableData: bigqueryApi?.tabledata);
     await _runFlutterTest(path.join(flutterRoot, 'dev', 'integration_tests', 'non_nullable'),
       options: <String>['--enable-experiment=non-nullable'],
-      skip: true, // TODO(jonahwilliams): re-enable after https://github.com/flutter/flutter/issues/55872
     );
     await _runFlutterTest(
       path.join(flutterRoot, 'dev', 'tracing_tests'),

packages/flutter_tools/lib/src/test/flutter_platform.dart

@@ -213,7 +213,7 @@ void catchIsolateErrors() {
 
 void main() {
   print('$_kStartTimeoutTimerMessage');
-  String serverPort = Platform.environment['SERVER_PORT'];
+  String serverPort = Platform.environment['SERVER_PORT'] ?? '';
   String server = Uri.decodeComponent('$encodedWebsocketUrl:\$serverPort');
   StreamChannel<dynamic> channel = serializeSuite(() {
     catchIsolateErrors();