docs: public void setFieldName(final String name)
      {
          if (!isInStruct())
          {
              throw new IllegalStateException("IonWriter.setFieldName() must be called before writing a value into a struct.");
          }
          if (name == null)
          {
              throw new NullPointerException("Null field name is not allowed.");
          }
          final SymbolToken token = intern(name);
          user.setFieldNameSymbol(token);
      }