Bug 25605: Replace NULL values with '' when downloading a report as a tab separated...

Bug 25605: Replace NULL values with '' when downloading a report as a tab separated text

Test plan :
    1- Create a new report : select * from items;
    2- Run it
    3- Download the report as a tab separated text
    4- There's a lot of 'use of uninitialized value in join or string at ...koha/reports/guided_reports.pl line xxx.'
    in plack-error.log
    5- Apply patch
    6- Restart all
    7- Redo 2 and 3
    8- No more warning.
    9- diff both downloaded files, they must be the same.

Signed-off-by: Marco Abi-Ramia <marco.abi-ramia@inlibro.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>