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/sampling/sampledSet/sampledSet/sampledSet.H

@@ -217,7 +217,7 @@ public:
         autoPtr<sampledSet> clone() const
         {
             NotImplemented;
-            return autoPtr<sampledSet>();
+            return nullptr;
         }
 
 

src/sampling/sampledSurface/sampledSurface/sampledSurface.C

@@ -137,7 +137,7 @@ Foam::tmp<Foam::scalarField> Foam::sampledSurface::sample
 ) const
 {
     NotImplemented;
-    return tmp<scalarField>();
+    return nullptr;
 }
 
 
@@ -147,7 +147,7 @@ Foam::tmp<Foam::vectorField> Foam::sampledSurface::sample
 ) const
 {
     NotImplemented;
-    return tmp<vectorField>();
+    return nullptr;
 }
 
 
@@ -157,7 +157,7 @@ Foam::tmp<Foam::sphericalTensorField> Foam::sampledSurface::sample
 ) const
 {
     NotImplemented;
-    return tmp<sphericalTensorField>();
+    return nullptr;
 }
 
 
@@ -167,7 +167,7 @@ Foam::tmp<Foam::symmTensorField> Foam::sampledSurface::sample
 ) const
 {
     NotImplemented;
-    return tmp<symmTensorField>();
+    return nullptr;
 }
 
 
@@ -177,7 +177,7 @@ Foam::tmp<Foam::tensorField> Foam::sampledSurface::sample
 ) const
 {
     NotImplemented;
-    return tmp<tensorField>();
+    return nullptr;
 }
 
 

src/sampling/sampledSurface/sampledSurface/sampledSurface.H

@@ -205,7 +205,7 @@ public:
         autoPtr<sampledSurface> clone() const
         {
             NotImplemented;
-            return autoPtr<sampledSurface>();
+            return nullptr;
         }
 
 

src/sampling/sampledSurface/sampledTriSurfaceMesh/sampledTriSurfaceMeshNormal.H

@@ -103,7 +103,7 @@ public:
         ) const
         {
             NotImplemented;
-            return tmp<scalarField>();
+            return nullptr;
         }
 
         //- Sample field on surface
@@ -119,7 +119,7 @@ public:
         ) const
         {
             NotImplemented;
-            return tmp<sphericalTensorField>();
+            return nullptr;
         }
 
         //- Sample field on surface
@@ -129,7 +129,7 @@ public:
         ) const
         {
             NotImplemented;
-            return tmp<symmTensorField>();
+            return nullptr;
         }
 
         //- Sample field on surface
@@ -139,7 +139,7 @@ public:
         ) const
         {
             NotImplemented;
-            return tmp<tensorField>();
+            return nullptr;
         }
 
         //- Interpolate field on surface
@@ -149,7 +149,7 @@ public:
         ) const
         {
             NotImplemented;
-            return tmp<scalarField>();
+            return nullptr;
         }
 
         //- Interpolate field on surface
@@ -165,7 +165,7 @@ public:
         ) const
         {
             NotImplemented;
-            return tmp<sphericalTensorField>();
+            return nullptr;
         }
 
         //- Interpolate field on surface
@@ -175,7 +175,7 @@ public:
         ) const
         {
             NotImplemented;
-            return tmp<symmTensorField>();
+            return nullptr;
         }
 
         //- Interpolate field on surface
@@ -185,7 +185,7 @@ public:
         ) const
         {
             NotImplemented;
-            return tmp<tensorField>();
+            return nullptr;
         }
 };
 

src/sampling/surfMeshSampler/surfMeshSampler/surfMeshSampler.H

@@ -181,7 +181,7 @@ public:
         autoPtr<surfMeshSampler> clone() const
         {
             NotImplemented;
-            return autoPtr<surfMeshSampler>();
+            return nullptr;
         }
 
 

src/sampling/surface/triSurfaceMesh/discreteSurfaceTemplates.C

@@ -120,10 +120,8 @@ Foam::discreteSurface::sampleType
         const VolFieldType& fld = mesh().lookupObject<VolFieldType>(fieldName);
         return sampleField(fld);
     }
-    else
-    {
-        return tmp<Field<Type>>();
-    }
+
+    return nullptr;
 }