Bug 7688: (follow-up) rewrite t/db_dependant/Serials.t

Bug 7688: (follow-up) rewrite t/db_dependant/Serials.t

A lot of tests were just a call to a funtion without any arguments,
whereas the function expect at least one argument.
These tests were kept, but all return values are now undef when a
mandatory argument is missing, so return values are consistent.

The part where subscription periodicity is changed could not work
because of ',' appended to each key in ModSubscription call. So it's
rewritten, taking into account the new API for subscription frequencies.
This script should leave your database intact because it revert any
modification made.

Also fix some warnings in C4::Serials and in C4::Items.
And fix a typo in koha-tmpl/.../subscription-numberpatterns.tt

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Comment: Work as described. No errors
Tested, again, without trouble.
With a suscription, tests successful.
No koha-qa errors

Signed-off-by: Galen Charlton <gmc@esilibrary.com>