[DS-1411] uncaught NPE in stats-log-converter -m

https://jira.duraspace.org/browse/DS-1411

"-m -i dspace.log" NPEs because File.getParentFile() returns null when there is no parent path.  Calculate absolute path first to avoid this.

Also handle the case of File.list() returning null.

Also tell the user if no files matched the pattern.