docs: private _Private_IonTextWriterBuilder fillDefaults()
      {
          // Ensure that we don't modify the user's builder.
          IonTextWriterBuilder b = copy();
  
          if (b.getCatalog() == null)
          {
              b.setCatalog(new SimpleCatalog());
          }
  
          if (b.getCharset() == null)
          {
              b.setCharset(UTF8);
          }
  
          return (_Private_IonTextWriterBuilder) b.immutable();
      }