C++11: Replaced the C NULL with the safer C++11 nullptr

applications/solvers/electromagnetics/mhdFoam/createPhiB.H

...leInterFoam/twoPhaseMixtureThermo/twoPhaseMixtureThermo.C

...InterFoam/multiphaseMixtureThermo/phaseModel/phaseModel.C

...tiphaseEulerFoam/multiphaseSystem/phaseModel/phaseModel.C

applications/solvers/multiphase/multiphaseEulerFoam/pEqn.H

...phase/multiphaseInterFoam/multiphaseMixture/phase/phase.C

...Systems/HeatTransferPhaseSystem/HeatTransferPhaseSystem.C

...aseSystems/phaseModel/MovingPhaseModel/MovingPhaseModel.C

...EulerFoam/phaseSystems/phaseModel/phaseModel/phaseModel.C

...EulerFoam/phaseSystems/phaseSystem/phaseSystemTemplates.C

...m/diameterModels/IATE/IATEsources/IATEsource/IATEsource.H

...m/diameterModels/IATE/IATEsources/IATEsource/IATEsource.H

...lvers/stressAnalysis/solidDisplacementFoam/createFields.H

applications/test/HashingSpeed/Test-HashingSpeed.C

applications/test/regex/Test-regex.C

...s/utilities/mesh/advanced/autoRefineMesh/autoRefineMesh.C

...cations/utilities/mesh/advanced/selectCells/selectCells.C

...lities/mesh/conversion/Optional/ccm26ToFoam/ccm26ToFoam.C

.../mesh/conversion/fluentMeshToFoam/extrudedQuadCellShape.C

...h/conversion/fluentMeshToFoam/extrudedTriangleCellShape.C