Bug 15061: Can't add item to rotating collection

Bug 15061: Can't add item to rotating collection

This patch prevents adding items to invalid (empty)
rotating collection id

To test:
1) Go to /cgi-bin/koha/rotating_collections/addItems.pl
2) Try to add an item, you get error show in attached pictures
3) Apply the patch
4) Go again to /cgi-bin/koha/rotating_collections/addItems.pl
5) Can't add any items

I don't know if this is a real problem

Followed test plan, works as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>