Adding empty non-terminals to zone.

Empty non-terminals are created after creating the zone list and
  before inserting into ZDS. The algorithm is quite straightforward
  - for each node it finds a 'parent' node and creates all nodes
    'missing' between the parent and the node. Search for parent
    optimized by saving the last parent + jumping directly to zone
    apex if number of common labes is equal to size of the zone
    apex' owner name.

Added function zn_is_empty().
Skipping empty nodes when processing non-authoritative nodes.