Add NO_LISTENERS constant

Author: mcculls

components/context/src/main/java/datadog/context/ThreadLocalContextManager.java

@@ -11,8 +11,10 @@ final class ThreadLocalContextManager implements ContextManager {
   private static final ThreadLocal<ContextHolder> CONTEXT_HOLDER =
       ThreadLocal.withInitial(ContextHolder::new);
 
+  private static final ContextListener[] NO_LISTENERS = {};
+
   private final Object listenersWriteLock = new Object();
-  volatile ContextListener[] listeners = {};
+  volatile ContextListener[] listeners = NO_LISTENERS;
 
   @Override
   public Context current() {
@@ -92,7 +94,7 @@ public void addListener(ContextListener listener) {
 
   void clearListeners() {
     synchronized (listenersWriteLock) {
-      listeners = new ContextListener[] {};
+      listeners = NO_LISTENERS;
     }
   }