Demote warning to debug message in TrackParticleCnvAlg for single photon sample

In cases where some tracks are reconstructed but the hard-scatter event does not have any charged particles leading to reconstructed tracks, the TrackParticleCnvAlg can throw a warning related to missing truth information. This typically happens in single photon sample with pile-up, for events with unconverted photons. The implementation is updated to throw a simple debug message in that case.

Closes ATLASRECTS-7342