Do not touch global forwarders from named.conf if there is no config in LDAP.
    
    Without this change, fwdtbl_update_requested variable in
    configure_zone_forwarders() was True even in cases where no forwarding
    was configured in named.conf and in LDAP.
    
    That would cause false positives in automatic empty zone unloading logic.
    
    https://fedorahosted.org/bind-dyndb-ldap/ticket/160