Bug 26175: Remove warn if undefined barcode in misc/export_records.pl

Bug 26175: Remove warn if undefined barcode in misc/export_records.pl

When using misc/export_records.pl --deleted_barcodes if barcode is NULL you get warn :
Use of uninitialized value in say at /home/koha/src/misc/export_records.pl

Also exporting barcode if empty string makes no sense.

Test plan :
1) Delete an item with barcode = NULL
2) Run : misc/export_records.pl --date=`date +%d/%m/%Y` --deleted_barcodes --filename=/tmp/deleted_barcodes
3) Check you se no warn "Use of uninitialized value in say ..."
4) Delete an item with barcode = ''
5) Run 2)
6) Check there is no empty line in /tmp/deleted_barcodes

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>