ENH: dummy I/O for nullObject, additional null subclass for zero/one

- the zero::null and one::null sub-classes add an additional null
  output adapter.

  The function of the nil class (special-purpose class only used for
  HashSet) is now taken by zero::null.

applications/test/HashSet/Test-hashSet.C

@@ -3,7 +3,7 @@
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
     \\  /    A nd           | Copyright (C) 2011 OpenFOAM Foundation
-     \\/     M anipulation  | Copyright (C) 2017 OpenCFD Ltd.
+     \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -26,6 +26,7 @@ Description
 \*---------------------------------------------------------------------------*/
 
 #include "hashedWordList.H"
+#include "nil.H"
 #include "HashSet.H"
 #include "Map.H"
 #include "labelPairHashes.H"
@@ -69,6 +70,8 @@ int main(int argc, char *argv[])
 
     Info<< "tableA keys: "; tableA.writeKeys(Info) << endl;
 
+    Info<< "tableB content: " << tableB << endl;
+
     auto keyIterPair = tableA.keys();
     for (const auto& i : keyIterPair)
     {