code: FieldNumberTree child(FieldDescriptorOrUnknown fieldDescriptorOrUnknown) {
      FieldNumberTree child =
          children.get(Key.fromFieldDescriptorOrUnknown(fieldDescriptorOrUnknown));
      return child == null ? EMPTY : child;
    }
docs: Returns the {@code FieldNumberTree} corresponding to this sub-field.
  
  <p>{@code empty()} if there is none.