docs: public ServiceReference<V> putReferenceIfAbsent(K key, ServiceReference<V> reference) {
          // If the key is null we can't do anything
          if (key == null)
              return null;
          if (reference == null) {
              // If the reference is null we can't add it to the map but we could still return an existing on if there was one so check this
              return getReference(key);
          }
  
          ConcurrentServiceReferenceElement<V> element = new ConcurrentServiceReferenceElement<V>(referenceName, reference);
          ConcurrentServiceReferenceElement<V> existingEntry = elementMap.putIfAbsent(key, element);
          if (existingEntry == null) {
              return null;
          } else {
              return existingEntry.getReference();
          }
      }