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/solvers/multiphase/compressibleMultiphaseInterFoam/multiphaseMixtureThermo/phaseModel/phaseModel.C

@@ -81,7 +81,7 @@ Foam::phaseModel::phaseModel
 Foam::autoPtr<Foam::phaseModel> Foam::phaseModel::clone() const
 {
     NotImplemented;
-    return autoPtr<phaseModel>();
+    return nullptr;
 }
 
 

applications/solvers/multiphase/interCondensatingEvaporatingFoam/temperaturePhaseChangeTwoPhaseMixtures/twoPhaseMixtureEThermo/twoPhaseMixtureEThermo.C

@@ -259,7 +259,7 @@ Foam::tmp<Foam::scalarField> Foam::twoPhaseMixtureEThermo::THE
 ) const
 {
     NotImplemented;
-    return tmp<Foam::scalarField>();
+    return nullptr;
 }
 
 
@@ -272,7 +272,7 @@ Foam::tmp<Foam::scalarField> Foam::twoPhaseMixtureEThermo::THE
 ) const
 {
     NotImplemented;
-    return tmp<Foam::scalarField>();
+    return nullptr;
 }
 
 
@@ -436,8 +436,8 @@ Foam::tmp<Foam::scalarField> Foam::twoPhaseMixtureEThermo::Cpv
 
 Foam::tmp<Foam::volScalarField> Foam::twoPhaseMixtureEThermo::CpByCpv() const
 {
-     NotImplemented;
-     return tmp<Foam::volScalarField>();
+    NotImplemented;
+    return nullptr;
 }
 
 
@@ -448,8 +448,8 @@ Foam::tmp<Foam::scalarField> Foam::twoPhaseMixtureEThermo::CpByCpv
     const label patchi
 ) const
 {
-     NotImplemented;
-     return tmp<Foam::scalarField>();
+    NotImplemented;
+    return nullptr;
 }
 
 

applications/solvers/multiphase/multiphaseEulerFoam/multiphaseSystem/phaseModel/phaseModel.C

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

applications/solvers/multiphase/multiphaseInterFoam/multiphaseMixture/phase/phase.C

@@ -68,7 +68,7 @@ Foam::phase::phase
 Foam::autoPtr<Foam::phase> Foam::phase::clone() const
 {
     NotImplemented;
-    return autoPtr<phase>();
+    return nullptr;
 }
 
 

applications/solvers/multiphase/reactingEulerFoam/phaseSystems/phaseModel/IsothermalPhaseModel/IsothermalPhaseModel.C

@@ -65,7 +65,7 @@ template<class BasePhaseModel>
 Foam::tmp<Foam::fvScalarMatrix>
 Foam::IsothermalPhaseModel<BasePhaseModel>::heEqn()
 {
-    return tmp<fvScalarMatrix>();
+    return nullptr;
 }
 
 

applications/solvers/multiphase/reactingEulerFoam/phaseSystems/phaseModel/MultiComponentPhaseModel/MultiComponentPhaseModel.C

@@ -152,7 +152,7 @@ Foam::MultiComponentPhaseModel<BasePhaseModel>::YiEqn
         )
     )
     {
-        return tmp<fvScalarMatrix>();
+        return nullptr;
     }
 
     const volScalarField& alpha = *this;

applications/solvers/multiphase/reactingEulerFoam/phaseSystems/phaseModel/PurePhaseModel/PurePhaseModel.C

@@ -57,8 +57,7 @@ Foam::PurePhaseModel<BasePhaseModel>::YiEqn
 )
 {
     NotImplemented;
-
-    return tmp<fvScalarMatrix>();
+    return nullptr;
 }
 
 

applications/solvers/multiphase/reactingEulerFoam/phaseSystems/phaseModel/phaseModel/phaseModel.C

@@ -77,7 +77,7 @@ Foam::phaseModel::phaseModel
 Foam::autoPtr<Foam::phaseModel> Foam::phaseModel::clone() const
 {
     NotImplemented;
-    return autoPtr<phaseModel>();
+    return nullptr;
 }
 
 

applications/solvers/multiphase/reactingEulerFoam/reactingTwoPhaseEulerFoam/twoPhaseSystem/diameterModels/IATE/IATEsources/IATEsource/IATEsource.H

@@ -115,7 +115,7 @@ public:
         autoPtr<IATEsource> clone() const
         {
             NotImplemented;
-            return autoPtr<IATEsource>();
+            return nullptr;
         }
 
 

applications/solvers/multiphase/twoPhaseEulerFoam/twoPhaseSystem/diameterModels/IATE/IATEsources/IATEsource/IATEsource.H

@@ -114,7 +114,7 @@ public:
         autoPtr<IATEsource> clone() const
         {
             NotImplemented;
-            return autoPtr<IATEsource>();
+            return nullptr;
         }