idmap: allow NULL domain sid for external mappings
    
    Since it is planned that the LDAP based ID providers (LDAP, AD, IPA)
    will always use libsss_idmap to map ID or get information about how to
    map it, it must be possible to add domains to libsss_idmap which do not
    have a SID or where is SID is not known when external mapping is used.
    Algorithmic mapping always requires a domain SID.
    
    Fixes https://fedorahosted.org/sssd/ticket/1960