Fix the reasons for warning from PP2MCP relation unpacker

There are warnings that make some tests fail

PP2MCPRelationU...WARNING Source location /Event/Spruce/HLT2/Rec/ProtoP/Charged not persisted or not unpacked, skipping relation.

or

PP2MCPRelationUnpacker#2            WARNING Source location /Event/Spruce/HLT2/Rec/ProtoP/Charged not persisted or not unpacked, skipping relation.

See eg https://lhcb-nightlies.web.cern.ch/nightly/lhcb-head/3485/DaVinci/x86_64_v2-centos7-gcc11-opt/tests