Bug 28385: Use ldapserver only if exists

Bug 28385: Use ldapserver only if exists

Fix test
t/db_dependent/Auth_with_ldap.t .. 1/4
 #   Failed test 'use C4::Auth_with_ldap;'
 #   at t/db_dependent/Auth_with_ldap.t line 123.
 #     Tried to use 'C4::Auth_with_ldap'.
 #     Error:  No ldapserver "id="ldapserver"" defined in KOHA_CONF: /etc/koha/sites/kohadev/koha-conf.xml at /kohadevbox/koha/C4/Auth_with_ldap.pm line 55.
 # Compilation failed in require at t/db_dependent/Auth_with_ldap.t line 123.
 # BEGIN failed--compilation aborted at t/db_dependent/Auth_with_ldap.t line 123.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>