ArcFilesReportGenerator does not handle unclosed ARCs properly

At BnF we use the arcfiles-report.txt report generated by ArcFilesReportGenerator, in an automated QA process that works with metadata ARC files.

In the case when an ARC file wouldn't have been properly closed (Heritrix crash or harvester shutdown), the generated report does not add the closure date nor the size, which makes our automated QA raise an exception.

So we should put default non-empty values in the report to have always the same number of fields per report line.