Fix broken null pointer test
Fixes bug that broke compilation with Gaudi v39:
CaloPi0Ntp.cpp:191:19: error: invalid conversion from 'int' to 'const LHCb::CaloCluster*'
Fixes bug that broke compilation with Gaudi v39:
CaloPi0Ntp.cpp:191:19: error: invalid conversion from 'int' to 'const LHCb::CaloCluster*'