public static @Nullable String readString(
    @NotNull IonReader reader,
    @Nullable String fieldName
  ) {
    IonType type = reader.next();
    assertFieldName(reader, fieldName);
    if (type == null) return null;
    return reader.stringValue();
  }
public static @Nullable String readString(
    @NotNull IonReader reader,
    @Nullable String fieldName
  )