Replaced NULL with nullptr in rdbms directory

rdbms/AutoRollback.cpp

rdbms/AutoRollback.hpp

rdbms/DbConnPoolTest.cpp

rdbms/DbRset.cpp

rdbms/DbRset.hpp

rdbms/NullDbValue.hpp

rdbms/OcciConn.cpp

rdbms/OcciConn.hpp

rdbms/OcciEnv.cpp

rdbms/OcciEnv.hpp

rdbms/OcciEnvSingleton.cpp

rdbms/OcciRset.cpp

rdbms/OcciRset.hpp

rdbms/OcciStmt.cpp

rdbms/PooledDbConn.cpp

rdbms/PooledDbConn.hpp

rdbms/SqliteConn.cpp

rdbms/SqliteRset.cpp

rdbms/SqliteRset.hpp

rdbms/SqliteStmt.cpp