docs: private _Private_IonBinaryWriterBuilder fillDefaults()
      {
          // Ensure that we don't modify the user's builder.
          _Private_IonBinaryWriterBuilder b = copy();
  
          if (b.getSymtabValueFactory() == null)
          {
              IonSystem system = IonSystemBuilder.standard().build();
              b.setSymtabValueFactory(system);
          }
  
          return b.immutable();
      }