LDAP Static and Dynamic Group mappings not properly checked against empty string

LDAP configuration requires all properties to be set for user and group configuration with `ldapUserMuliGroup`.  The LDAP Reference class does not properly check for empty string against null when configured with the template.

Guards:

https://github.com/nuxeo/nuxeo/blob/master/nuxeo-services/nuxeo-platform-directory/nuxeo-platform-directory-ldap/src/main/java/org/nuxeo/ecm/directory/ldap/LDAPReference.java#L525

https://github.com/nuxeo/nuxeo/blob/master/nuxeo-services/nuxeo-platform-directory/nuxeo-platform-directory-ldap/src/main/java/org/nuxeo/ecm/directory/ldap/LDAPReference.java#L141

The workaround is to set a valid attribute name for the static & dynamic properties.  LDAP setup with mutli LDAP group should have all fields required on setup.