* [BUGFIX] Fix crash when no emails or groups are retrieved from LDAP.
If group or email attribute configured by user in configuration is not
found in user object the list of attributes in LDAP search result is empty.
This change introduces a check before accessing the first element of the
list which previously led to out of bound access.
Fixes#647.
* [MISC] Change log level of LDAP connection creation to trace.
- Make the DN concatenation uniform between both Users and Groups
- Make it possible to use a blank or commented out additional_users_dn or additional_groups_dn for ldap backends
- Fixes#508
