Avoid allocating empty lists

Summary: Changelog: [Internal] Optimize Fabric memory allocations on Android

Reviewed By: genkikondo

Differential Revision: D36098912

fbshipit-source-id: 8d34a652db019865a8306bc070a08f5ddfef96bf

ReactAndroid/src/main/java/com/facebook/react/fabric/mounting/MountItemDispatcher.java

@@ -356,13 +356,16 @@ public class MountItemDispatcher {
   @Nullable
   private static <E extends MountItem> List<E> drainConcurrentItemQueue(
       ConcurrentLinkedQueue<E> queue) {
+    if (queue.isEmpty()) {
+      return null;
+    }
     List<E> result = new ArrayList<>();
-    while (!queue.isEmpty()) {
+    do {
       E item = queue.poll();
       if (item != null) {
         result.add(item);
       }
-    }
+    } while (!queue.isEmpty());
     if (result.size() == 0) {
       return null;
     }