Fix logic error in FunctionalDiElectronMaker

Fix bug in the logic of the FunctionalDiElectronMaker which was causing failed combinations to be added to the output container (with NULL end vertices).

Thanks to @dmitzel for reporting, this should solve the warnings reported here.

Change also the input type of FunctionalDiElectronMaker, FunctionalBremAdder and ParticleWithBremMaker from LHCb::Particles to LHCb::Particle::Range so both LHCb::Particle::Container and LHCb::Particle::Selection are accepted, following this suggestion.