Bug 26302: (QA follow-up) Prevent empty call numbers

Bug 26302: (QA follow-up) Prevent empty call numbers

This does not make the code easier, but here we go:

Suppose resultsMaxItems == 2.
If we would normally list two call numbers like:
    Liberty (2) [Call number: PERL F 1, PERL F 2].
If one call number would be empty, we now list:
    Liberty (2) [Call number: PERL F 1, ...].
And when both are empty, we only show a number:
    Liberty (2).

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>