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.

src/meshTools/coordinateSystems/coordinateRotation/EulerCoordinateRotation.C

@@ -70,7 +70,7 @@ Foam::tmp<Foam::vectorField> Foam::EulerCoordinateRotation::transform
 ) const
 {
     NotImplemented;
-    return tmp<vectorField>();
+    return nullptr;
 }
 
 
@@ -80,7 +80,7 @@ Foam::tmp<Foam::vectorField> Foam::EulerCoordinateRotation::invTransform
 ) const
 {
     NotImplemented;
-    return tmp<vectorField>();
+    return nullptr;
 }
 
 
@@ -96,8 +96,8 @@ Foam::tmp<Foam::tensorField> Foam::EulerCoordinateRotation::transformTensor
     const tensorField& st
 ) const
 {
-     NotImplemented;
-    return tmp<tensorField>();
+    NotImplemented;
+    return nullptr;
 }
 
 
@@ -117,7 +117,7 @@ Foam::tmp<Foam::tensorField> Foam::EulerCoordinateRotation::transformTensor
 ) const
 {
     NotImplemented;
-    return tmp<tensorField>();
+    return nullptr;
 }
 
 

src/meshTools/coordinateSystems/coordinateRotation/STARCDCoordinateRotation.C

@@ -71,7 +71,7 @@ Foam::tmp<Foam::vectorField> Foam::STARCDCoordinateRotation::transform
 ) const
 {
     NotImplemented;
-    return tmp<vectorField>();
+    return nullptr;
 }
 
 
@@ -81,7 +81,7 @@ Foam::tmp<Foam::vectorField> Foam::STARCDCoordinateRotation::invTransform
 ) const
 {
     NotImplemented;
-    return tmp<vectorField>();
+    return nullptr;
 }
 
 
@@ -97,8 +97,8 @@ Foam::tmp<Foam::tensorField> Foam::STARCDCoordinateRotation::transformTensor
     const tensorField& st
 ) const
 {
-     NotImplemented;
-    return tmp<tensorField>();
+    NotImplemented;
+    return nullptr;
 }
 
 
@@ -118,7 +118,7 @@ Foam::tmp<Foam::tensorField> Foam::STARCDCoordinateRotation::transformTensor
 ) const
 {
     NotImplemented;
-    return tmp<tensorField>();
+    return nullptr;
 }
 
 

src/meshTools/coordinateSystems/coordinateRotation/axesRotation.C

@@ -245,7 +245,7 @@ Foam::tmp<Foam::tensorField> Foam::axesRotation::transformTensor
 ) const
 {
     NotImplemented;
-    return tmp<tensorField>();
+    return nullptr;
 }
 
 
@@ -265,7 +265,7 @@ Foam::tmp<Foam::tensorField> Foam::axesRotation::transformTensor
 ) const
 {
     NotImplemented;
-    return tmp<tensorField>();
+    return nullptr;
 }
 
 

src/meshTools/searchableSurfaces/searchableSurface/searchableSurface.H

@@ -139,7 +139,7 @@ public:
         virtual autoPtr<searchableSurface> clone() const
         {
             NotImplemented;
-            return autoPtr<searchableSurface>();
+            return nullptr;
         }
 
 

src/meshTools/sets/topoSetSource/topoSetSource.H

@@ -243,7 +243,7 @@ public:
         autoPtr<topoSetSource> clone() const
         {
             NotImplemented;
-            return autoPtr<topoSetSource>();
+            return nullptr;
         }
 
 

src/meshTools/sets/topoSets/topoSet.H

@@ -254,7 +254,7 @@ public:
         autoPtr<topoSet> clone() const
         {
             NotImplemented;
-            return autoPtr<topoSet>();
+            return nullptr;
         }