Bug 33716: (QA follow-up) Update z39.50 availability search in 'Request from partners...

Bug 33716: (QA follow-up) Update z39.50 availability search in 'Request from partners' screen

Test plan:
- Enable ILLCheckAvailability;
- Enable ILLModule and install FreeForm
- Update partner_code in koha-conf.xml to match the patron category being used ('IL' if on k-t-d)
- Install and configure z39.50 availability plugin, link it to the patron ID ('16' if on k-t-d)
- Add a valid e-mail address to the patron of 'IL' category
- Create a new ILL request and go to 'Manage request' screen
- Click 'Place request with partners' notice the label 'Partners available for searching: none'
- Apply patch and refresh, notice a 'Search selected partners' button now shows up after you select the partner from the list

This is a fix for a regression caused by the rest of the code in this bug. I'm submitting this here since this has not yet been pushed and the fix is quite self-explanatory

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>