Avoid allocating FieldSet iterator if FieldSet is empty

This saves some allocations during writing out messages with extensions declared in the common case of empty FieldSet.

PiperOrigin-RevId: 636031501

java/core/src/main/java/com/google/protobuf/FieldSet.java

@@ -213,6 +213,10 @@ private static <T extends FieldDescriptorLite<T>> void cloneFieldEntry(
    * library as it is not protected from mutation when fields is not immutable.
    */
   public Iterator<Map.Entry<T, Object>> iterator() {
+    // Avoid allocation in the common case of empty FieldSet.
+    if (isEmpty()) {
+      return Collections.emptyIterator();
+    }
     if (hasLazyField) {
       return new LazyIterator<T>(fields.entrySet().iterator());
     }
@@ -225,6 +229,10 @@ public Iterator<Map.Entry<T, Object>> iterator() {
    * fields is not immutable.
    */
   Iterator<Map.Entry<T, Object>> descendingIterator() {
+    // Avoid an allocation in the common case of empty FieldSet.
+    if (isEmpty()) {
+      return Collections.emptyIterator();
+    }
     if (hasLazyField) {
       return new LazyIterator<T>(fields.descendingEntrySet().iterator());
     }