Bug 29883: avoid uninitialized value warn in GetAuthorisedValues sub

Bug 29883: avoid uninitialized value warn in GetAuthorisedValues sub

GetAuthorisedValues is defined with optional parameter $category but it
is instantly interpolated without preventing "Use of uninitialized value
$category in concatenation (.) or string at .../C4/Koha.pm line 491."
warning.
As category param is optional, we can avoid throwing that warn as it is
something that can happen and is not an actual error:
C<$category> returns authorized values for just one category (optional).

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>