Add Bkg_TruthEvent to the list of known McEventCollection names in HepMcParticleLink.cxx

Pre-mixing jobs use Bkg_TruthEvent rather than TruthEvent as the output McEventCollection name. HepMcParticleLink doesn't know that this is a possible collection name that it should check for so all links are broken and the attempts to find the McEventCollection in the T->P converter methods always return nullptr. This causes the above crash as there is currently no protection against dereferencing a null McEventCollection pointer. Adding Bkg_TruthEvent to the list of known McEventCollection names in HepMcParticleLink.cxx should fix ATLASSIM-4539.