avoid comparing SmartRef to 0, use nullptr instead

Event/RecEvent/src/ProtoParticle.cpp

@@ -71,7 +71,7 @@ LHCb::ProtoParticle::ExtraInfo::size_type LHCb::ProtoParticle::clearCalo( const
   LHCb::ProtoParticle::ExtraInfo::size_type removed = 0;
 
   // remove NULL hypos
-  const auto it = std::remove( m_calo.begin(), m_calo.end(), 0 );
+  const auto it = std::remove( m_calo.begin(), m_calo.end(), nullptr );
   if ( m_calo.end() != it ) {
     removed += m_calo.end() - it;
     m_calo.erase( it, m_calo.end() );