Bug 12003: Do not calculate next pubdate for irregular subscriptions

Show 'Unknown' when planneddate and publisheddate cannot be calculated

Also fixes SQL query in misc/cronjobs/serialsUpdate.pl that was still
using "periodicity != 32" to exclude irregular subscriptions from
results

Test plan:

1) Create a subscription in the serials module. Make sure to choose:
   Frequency = Irregular
2) Test the prediction pattern, first publication date is set to
   "First issue publication date" field, others will show as
   'unknown'
3) Save the subscription
4) Check the created issue - it will show a published date and a
   planned date (same as "First issue publication date" field)
5) Receive the issue and check the next generated issue, planned
   date and published date should show as 'Unknown'
6) Generate a next issue, planned date and published date should
   also show as 'Unknown'

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Work as described following test plan.
No koha-qa errors

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Passes all tests and QA script.
Also tested:
- multi receiving generates mulitple issues without dates - 'unknown'
- staff detail page shows the dates empty, which is fine
- OPAC detail page shows the dates empty, which is fine
- serial collection page shows 'unknown' and those issues appear
  on the 'manage' tab, as they did in the past
- Editing the issue from the serial collection page leaves the
  date fields empty.
- Receving the issue, setting the status to 'Arrived' the Expected on
  date is set to 'today' automatically. Date published has to be
  entered manually (maybe something we could improve later
- subscription detail > issues tab shows Uknown.
- t/db_dependent/Serials/GetNextDate.t pass.

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