Fabric: Small optimization in RCTMountingManager

Summary: `pullTransaction` can return an empty transaction. That's fine to interate over the empty collection and do nothing, but it's inneficient because we also call delegate methods around the loop (that can be expensive).

Reviewed By: sammy-SC

Differential Revision: D17053430

fbshipit-source-id: c78959d47ea22cd9bb99419f6a80de3ac8e89fd3

React/Fabric/Mounting/RCTMountingManager.mm

@@ -251,10 +251,15 @@ static void RNPerformMountInstructions(ShadowViewMutationList const &mutations,
   }
 
   auto surfaceId = transaction->getSurfaceId();
+  auto &mutations = transaction->getMutations();
+
+  if (mutations.size() == 0) {
+    return;
+  }
 
   RCTAssertMainQueue();
   [self.delegate mountingManager:self willMountComponentsWithRootTag:surfaceId];
-  RNPerformMountInstructions(transaction->getMutations(), self.componentViewRegistry);
+  RNPerformMountInstructions(mutations, self.componentViewRegistry);
   [self.delegate mountingManager:self didMountComponentsWithRootTag:surfaceId];
 }