Use nullptr instead of NULL in C++ code

ReleaseNotes.md

catalogue/CreateAdminUserCmdLineArgsTest.cpp

catalogue/CreateSchemaCmdLineArgsTest.cpp

catalogue/DropSchemaCmdLineArgsTest.cpp

catalogue/DummyCatalogue.hpp

catalogue/PollDatabaseCmdLineArgsTest.cpp

catalogue/VerifySchemaCmdLineArgsTest.cpp

cmdline/restore_files/RestoreFilesCmdLineArgs.cpp

common/Configuration.cpp

common/Configuration.hpp

common/SmartArrayPtr.hpp

common/SmartFILEPtr.cpp

common/SmartFILEPtr.hpp

common/SmartFd.cpp

common/SmartFd.hpp

common/checksum/ChecksumBlob.cpp

common/dataStructures/TapeDrive.hpp

common/exception/Backtrace.cpp

common/exception/Backtrace.hpp

common/exception/Errnum.cpp