Or, you could write a plain text file.

Yes, that means the FS will sometimes punch nulls towards the tail of the log. However, it is the lowest latency / write amplification way to get stuff on disk (other than a blocked compression format, which would be a small change to syslog), so if the text file gets holes punched in it, the journalctl file would be truncated before the hole anyway in practice.

If you really care about nulls in logs for ideological reasons, you could write a few lines of code that finds the first stream of nulls in the text file, then truncates there.

In practice, no one wants that. It is strictly worse than returning partial entries after the hole, and by the time you are hitting this corner case, you are debugging a kernel crash.