Bug 29736: Don't return empty clubs

Bug 29736: Don't return empty clubs

There is an error when placing a hold for a club without members:
Uncaught TypeError: err.responseJSON.error is undefined

It seems that we should remove clubs without members from the search.

Test plan:
Create 1 club xx with 2 patrons
Create 1 club xxx with 1 patron and cancel their enrolment
Create 1 club xxxx without patron

Place a hold for club "x", only the first one should be returned with
this patch.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>