update: replace null with empty list parameter in async decision method for consistency

Author: FarhanAnjum-opti

core-api/src/test/java/com/optimizely/ab/OptimizelyUserContextTest.java

@@ -2562,7 +2562,7 @@ public void decideAsync_featureTest() throws InterruptedException {
         user.decideAsync(flagKey, decision -> {
             result[0] = decision;
             latch.countDown();
-        }, null);
+        }, Collections.emptyList());
 
         assertTrue(latch.await(1, java.util.concurrent.TimeUnit.SECONDS));
         OptimizelyDecision decision = result[0];