code: @SuppressWarnings(value = "RCN_REDUNDANT_NULLCHECK_OF_NONNULL_VALUE",
        justification = "Findbugs believes readRecord(null) is non-null. This is not true.")
    protected RecordEnvelope<D> readRecordEnvelopeImpl() throws DataRecordException, IOException {
      D record = readRecordImpl(null);
      return  record == null ? null : new RecordEnvelope<>(record);
    }
docs: Subclasses should implement this or {@link #readRecordImpl(Object)}
  instead of {@link org.apache.gobblin.source.extractor.Extractor#readRecord}