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/generation/foamyMesh/conformalVoronoiMesh/DelaunayMesh/DistributedDelaunayMesh.C

@@ -794,14 +794,12 @@ Foam::DistributedDelaunayMesh<Triangulation>::distribute
 {
     if (!Pstream::parRun())
     {
-        return autoPtr<mapDistribute>();
+        return nullptr;
     }
 
     distributeBoundBoxes(decomposition.procBounds());
 
-    autoPtr<mapDistribute> mapDist = decomposition.distributePoints(points);
-
-    return mapDist;
+    return decomposition.distributePoints(points);
 }
 
 

applications/utilities/mesh/generation/foamyMesh/conformalVoronoiMesh/searchableSurfaceFeatures/searchableSurfaceFeatures.H

@@ -101,7 +101,7 @@ public:
         virtual autoPtr<searchableSurfaceFeatures> clone() const
         {
             NotImplemented;
-            return autoPtr<searchableSurfaceFeatures>();
+            return nullptr;
         }
 
 
@@ -139,7 +139,7 @@ public:
         //- Return an extendedFeatureEdgeMesh containing the features
         virtual autoPtr<extendedFeatureEdgeMesh> features() const
         {
-            return autoPtr<extendedFeatureEdgeMesh>();
+            return nullptr;
         }
 };
 

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;
         }