Bug 25440: Fix for "uninitialized value in hash" warning in smart-rules.pl

Bug 25440: Fix for "uninitialized value in hash" warning in smart-rules.pl

This warning emitted:

Use of uninitialized value in hash element
at /admin/smart-rules.pl line 569.

that happened because we have NULLs in SQL results for 'categorycode'
and 'itemtype' which later used as 'any' kind of category/item in the
template, so for the template it passed this way:

    $rules->{ $r->{categorycode} }->{ $r->{itemtype} }->...

but undef will stringify as "" to become a hash key ("Hashes are
unordered collections of scalar values indexed by their associated
string key" https://perldoc.perl.org/perldata.html),

that's why "undef warning". To prevent warning here is the simple fix:

    $rules->{ $r->{categorycode} // '' }->{ $r->{itemtype} // '' }->...

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>