autovacuum `null` usename causes monitor_db_sessions to exit loop

[gabrielwol]

[2026-01-08 19:40:10] ERROR - Task failed with exception source=task loc=task_runner.py:1008
NotNullViolation: null value in column "usename" of relation "session_log" violates not-null constraint
DETAIL:  Failing row contains (7173, null, , 2026-01-08 19:04:14, active, Timeout: VacuumDelay, {}, autovacuum: VACUUM ANALYZE here_analysis.transport_canada_sample..., 2026-01-08 19:04:14, 2026-01-08 19:04:14, 2026-01-08 19:04:14, autovacuum worker, 35.92, null, 2026-01-07).