unset _pulsar on complete

to make sure we can copy.deepcopy the report

@daniel-eggert this resolves our discussion in https://git.geomar.de/digital-earth/dasf/dasf-messaging-python/-/merge_requests/37#note_40839 The problem is, that pydantic makes a deepcopy of the objects and for the progress report, this means that the PulsarMessageConsumer is deepcopied as well which seems to be not a good idea. That is why the _pulsar is now set to None when the report is marked as complete.