Avoid duplicate exceptions on VM Service Mock failure (#10197)

On failure to configure a mock VM service, we get a useful exception in
setUpAll(). This change prevents an additional failure in tearDownAll()
that provides no additional useful diagnostic info.

packages/flutter_tools/test/devfs_test.dart

@@ -380,7 +380,7 @@ class MockVMService extends BasicMock implements VMService {
   }
 
   Future<Null> tearDown() async {
-    await _server.close();
+    await _server?.close();
   }
 
   @override