Bug 18312: Fix export unless a file is supplied

Bug 18312: Fix export unless a file is supplied

Bug 18087 breaks export unless a file is supplied.

Can't use an undefined value as a HASH reference at
/home/vagrant/kohaclone/tools/export.pl line 75.

Test plan:
Export records using a file of id that is not a valid file (not txt or
csv)
Export records using a valid file
Export records without supplying a file

=> The export should work or fail as expected.

Signed-off-by: Jesse Maseto <jesse@bywatersolutions.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Mason James <mtj@kohaaloha.com>