Bug 10872 - C4::Items GetHiddenItems fix and optimization

Bug 10872 - C4::Items GetHiddenItems fix and optimization

There should be a:
"require YAML;"
or
"use YAML qw/Load/;"

as the GetHiddenItems routine has a reference to YAML::Load.

This was discovered while adding a GetHiddenItems() call into
opac/opac-MARCdetail.pl. I believe this problem dates back to
bug 6488 or bug 5984.

I also added an optimization to GetHiddenItems to prevent
processing if there is nothing in the system preference. Test
by searching for a biblio which has some or all of its items
hidden.

Signed-off-by: Mason James <mtj@kohaaloha.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Maybe
  return () unless $yaml =~ /\S/;
or
  return () if $yaml =~ /^\s*$/;
would have been easier to read.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>