Bug 9299: fix LDAP login breakage when ExtendedPatronAttributes is on

Bug 9299: fix LDAP login breakage when ExtendedPatronAttributes is on

To test:
- Configure an LDAP server and $KOHA_CONF, etc.
- Make sure ExtendedPatronAttributes is defined and that
  there is no attribute defined that is specified to be
  a unique ID.
- Try to log in using an account originating from the
  LDAP directory.
- You will got a software error:
  Can't use an undefined value as an ARRAY reference at
  /home/koha/src/C4/Auth_with_ldap.pm line 183.
- Apply the patch.
- Try to log in again; this time it should work.

Signed-off-by: Nuño López Ansótegui <nunyo@masmedios.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>