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/OpenFOAM/algorithms/dynamicIndexedOctree/dynamicIndexedOctree.C

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           |
+    \\  /    A nd           | Copyright (C) 2019 OpenCFD Ltd.
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
                             | Copyright (C) 2011-2016 OpenFOAM Foundation
@@ -2400,7 +2400,7 @@ const Foam::labelList& Foam::dynamicIndexedOctree<Type>::findIndices
         return *(contents_[getContent(contentIndex)]);
     }
 
-    return Foam::emptyLabelList;
+    return labelList::null();
 }