STYLE: replace zero::null usage with zero

- the null output adapter was previously used for the HashTables API
  when HashSet actually stored key/value. Now that the node only
  contains the key, having suppressed output is redundant, as is the
  zero::null class (reduces clutter)

STYLE: replace one::minus dispatch in extendedEdgeMesh

GIT: remove Foam::nil typedef (deprecated since May-2017)