LP#1667835: avoid edi_fetcher.pl crash upon fetching zero-length file

LP#1667835: avoid edi_fetcher.pl crash upon fetching zero-length file

This patch fixes an issue that could cause edi_fetcher.pl to crash
if the EDI partner supplies a zero-length file for Net::FTP to
download; such files are now skipped.

Testing would entail setting up an FTP server that contains a
zero-length file, then setting up an EDI remote account and
using edi_fetcher.pl to try to retrieve the file.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>