Fix issue inserting the empty log row

If the job started, and then was restarted, the row will already exist. So
don't error on a conflict.

guix-data-service/jobs/load-new-guix-revision.scm

@@ -118,7 +118,7 @@
   (exec-query
    conn
    "INSERT INTO load_new_guix_revision_job_logs (job_id, contents) VALUES
-($1, NULL)"
+($1, NULL) ON CONFLICT DO NOTHING"
    (list job-id)))
 
 (define (combine-log-parts! conn job-id)