phoenix_event_data_dump test fails with dd4hep

See e.g. https://lhcb-nightlies.web.cern.ch/nightly/lhcb-master/2115/Moore/x86_64_v2-centos7-gcc12-dbg/tests#RecoConf_phoenix_event_data_dump

| JSON format error | Expected the event 'EVENT-268773-4179748', collection 'LongTracks' to be of type "list", found NoneType |

Not really sure why, test log doesn't show any obvious issues.

@bcouturi @sponce Please take a look.