Bug 28012: Use definedness test instead of bool when checking whether to null fields

Bug 28012: Use definedness test instead of bool when checking whether to null fields

Saving a new numbering pattern didn't work without having to fill all
the fields, even those that are not mandatory.

To test:
 1) Go to /cgi-bin/koha/serials/subscription-numberpatterns.pl
    and try creating a new pattern, notice that only Name and
    Description are mandatory after applying this patch
 2) Make sure editing existing numbering patterns still works

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>