Bug 13085: bugfix to allow call of export.pl in commandline without id_list_file...

Bug 13085: bugfix to allow call of export.pl in commandline without id_list_file parameter

Test plan
1/ run ./tools/export.pl
2/ notice an error like cannot open 0: No such file or directory at ./tools/export.pl line 209.
3/ apply patch
4/ run again
5/ no errors

Signed-off-by: Chris <chris@bigballofwax.co.nz>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Works as described, adding an additional check.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>