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/sizeof/Test-sizeof.C

@@ -61,6 +61,10 @@ int main(int argc, char *argv[])
         nil x;
         cout<<"nil:" << sizeof(x) << nl;
     }
+    {
+        zero x;
+        cout<<"zero:" << sizeof(x) << nl;
+    }
     {
         bool x(0);
         cout<<"bool:" << sizeof(x) << nl;
@@ -93,6 +97,10 @@ int main(int argc, char *argv[])
         cout<<"double:" << sizeof(double) << nl;
     }
 
+    {
+        cout<<"string:" << sizeof(Foam::string) << nl;
+    }
+
 
     Info << "---\nEnd\n" << endl;