Restore early return to NativeAnimatedHelper.flushQueue

Summary:
Brings back a small performance win of avoiding starting/stopping a batch if the queue is empty.

## Background

This was attempted previously but it happened to expose a single frame animation visual regression in VR.

1. First added in D36298399 (55ee8ce0c4), reverted in D36378363
2. Added in a different form in D36338606 (35e2a63b8d), reverted in D36479089 (a4690d054f)
3. Root cause of the visual regressions introduced was fixed in D36612758 (40f4c662bc)

Now that we've fixed the root cause of the visual regression, this is safe to restore.

Changelog: [Internal]

Reviewed By: JoshuaGross

Differential Revision: D36670591

fbshipit-source-id: 42b6bc9c7764484f2dbb9edb122780a91b7393b4

Libraries/Animated/NativeAnimatedHelper.js

@@ -139,6 +139,14 @@ const API = {
     invariant(NativeAnimatedModule, 'Native animated module is not available');
     flushQueueTimeout = null;
 
+    // Early returns before calling any APIs
+    if (useSingleOpBatching && singleOpQueue.length === 0) {
+      return;
+    }
+    if (!useSingleOpBatching && queue.length === 0) {
+      return;
+    }
+
     if (useSingleOpBatching) {
       // Set up event listener for callbacks if it's not set up
       if (