Bug 15324: Do not display 'Waiting holds:" if no holds are waiting

 Bug 15324: Do not display 'Waiting holds:" if no holds are waiting

Test plan:
- Make sure that you have a patron A with waiting holds and a patron B
  without waiting holds
- Go to the checkout page for patron A (Home > Circulation > Checkouts
> ...)
- At the right, there is a title "Waiting holds:" and a list of
  holds waiting
- Go to the checkout page for patron B
=> Without this patch "Waiting holds:" is displayed

Sign-off on Jonathan's patch
Signed-off-by: Marc Véron <veron@veron.ch>
http://bugs.koha-community.org/show_bug.cgi?id=14324

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>
(cherry picked from commit cd46c5dc75f9381d4766b0bd70b3d089e53e539b)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>