STYLE: use 'return nullptr' for empty autoPtr/tmp returns

- both autoPtr and tmp are defined with an implicit construct from
  nullptr (but with explicit construct from a pointer to null).
  Thus is it safe to use 'nullptr' when returning an empty autoPtr or tmp.

applications/utilities/mesh/manipulation/createBaffles/faceSelection/faceSelection.H

@@ -109,7 +109,7 @@ public:
         autoPtr<faceSelection> clone() const
         {
             NotImplemented;
-            return autoPtr<faceSelection>();
+            return nullptr;
         }
 
 

applications/utilities/mesh/manipulation/createBaffles/faceSelection/faceZoneSelection.H

@@ -86,7 +86,7 @@ public:
         autoPtr<faceSelection> clone() const
         {
             NotImplemented;
-            return autoPtr<faceSelection>();
+            return nullptr;
         }
 
 

applications/utilities/mesh/manipulation/createBaffles/faceSelection/searchableSurfaceSelection.H

@@ -79,7 +79,7 @@ public:
         autoPtr<faceSelection> clone() const
         {
             NotImplemented;
-            return autoPtr<faceSelection>();
+            return nullptr;
         }