Bug 9574: Plack fixes for serials/subscription-add.pl

Bug 9574: Plack fixes for serials/subscription-add.pl

This patch fixes the following warnings under Plack:
* Use of uninitialized value $firstissuedate in string eq at \
    /home/jcamins/kohaclone/serials/subscription-add.pl line 92.
* Use of uninitialized value $firstissuedate in substr at \
    /home/jcamins/kohaclone/serials/subscription-add.pl line 105.
* Use of qw(...) as parentheses is deprecated at \
    /home/jcamins/kohaclone/serials/subscription-add.pl line 95.
* Variable "$firstissuedate" is not available at \
    /home/jcamins/kohaclone/serials/subscription-add.pl line 338.

To test:
1) Create a subscription. Ensure that setting the first issue's
   publication date works.
2) Edit a subscription. Ensure that the first issue's publication
   date shows up.
3) Sign off.

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
No regressions found, all tests and QA script pass.
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>