noop listener in testcase for onMetricsSnapshot

Author: Robin Duda

core/test/java/com/codingchili/core/context/SystemContextTest.java

@@ -25,7 +25,7 @@
 @RunWith(VertxUnitRunner.class)
 public class SystemContextTest {
     private SystemContext context;
-    private OnMetricListener listener;
+    private OnMetricListener listener = metrics -> { };
     private SystemSettings settings;
 
     @Rule