Fix LDAP template to take into account user search filter

If a user search filter is filled in the LDAP configuration, it adds an empty searchFilter in the configuration file

        <searchFilter></searchFilter>

instead of

        <searchFilter>(objectClass=user)</searchFilter>

Running a request will then fail with

Caused by: javax.naming.directory.InvalidSearchFilterException: Missing 'equals'; remaining name 'ou=Users,dc=example,dc=com'