ENH: code adjustments for updated NullObject

- can now safely use labelList::null() instead of emptyLabelList for
  return values. No special treatment required for lists.

  Possible replacements:

      if (notNull(list) && list.size())  ->  if (list.size())
      if (isNull(list) || list.empty())  ->  if (list.empty())

  The receiver may still wish to handle differently to distinguish
  between a null list and an empty list, but no additional special
  protection is required when obtaining sizes, traversing, outputting
  etc.

src/sampling/meshToMesh/distributedWeightedFvPatchFieldMapper.H

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2015 OpenCFD Ltd.
+    \\  /    A nd           | Copyright (C) 2015-2019 OpenCFD Ltd.
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
                             | Copyright (C) 2015-2016 OpenFOAM Foundation
@@ -79,11 +79,12 @@ public:
             weights_(weights),
             hasUnmapped_(false)
         {
-            forAll(addressing_, i)
+            for (const labelList& addr : addressing)
             {
-                if (addressing_[i].size() == 0)
+                if (addr.empty())
                 {
                     hasUnmapped_ = true;
+                    break;
                 }
             }
 
@@ -99,8 +100,7 @@ public:
         }
 
     //- Destructor
-    virtual ~distributedWeightedFvPatchFieldMapper()
-    {}
+    virtual ~distributedWeightedFvPatchFieldMapper() = default;
 
 
     // Member Functions