TempDB Version Store used by DB STARTUP background process
I checked if in other enviroments the same query on dm_tran_active_snapshot_database_transactions returns the same results and no! It isn't normal having DB STARTUP snapshot active from days.
I checked the errorlog of 13 days ago and I found errors about an unexpected shutdown and failover.
So I tried to remove the database with DBID 7 from the availability group and I re-run the two queries: Now tempdb and version store are empty and the sessions with transaction_id 0 are gone.
I re-added the database to his availability group and I resolved the issue.