Bug 22944: avoid AnonymousPatron in search_patrons_to_anonymise

Bug 22944: avoid AnonymousPatron in search_patrons_to_anonymise

In Koha::Patrons::search_patrons_to_anonymise() old issues already affected to the anonymous patron should be avoided. It is useless and can impact performance.

It is already working if AnymousPatron is not defined or 0 because of :
  'old_issues.borrowernumber' => { 'not' => undef }

In theory the anymous patron should have privacy=0 but in case its not it should be explicitly avoided.

Test plan :
Run t/db_dependent/Koha/Patrons.t

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>