protobuf: fix a data race in AbstractProtobufList

Unconditional update of isMutable causes data races for
LazyStringArrayList::EMPTY_LIST that is used as the default value
for protobuf fields.

Not writing to it unconditionally may also improve performance
by reducing contention for the presumably widely shared EMPTY_LIST object.

PiperOrigin-RevId: 510366298

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

@@ -130,8 +130,10 @@ abstract class AbstractProtobufList<E> extends AbstractList<E> implements Protob
 
   @Override
   public final void makeImmutable() {
+    if (isMutable) {
       isMutable = false;
     }
+  }
 
   @Override
   public E remove(int index) {