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/finiteArea/interpolation/edgeInterpolation/edgeInterpolationScheme/edgeInterpolationScheme.H

@@ -189,7 +189,7 @@ public:
         virtual tmp<GeometricField<Type, faePatchField, edgeMesh>>
         correction(const GeometricField<Type, faPatchField, areaMesh>&) const
         {
-            return tmp<GeometricField<Type, faePatchField, edgeMesh>>();
+            return nullptr;
         }
 
         //- Return the face-interpolate of the given cell field