PR #17492 fixed `markAsMessagesListFetchOngoing` to set
`syncStageStartedAt`, but also changed `isSyncStale` to return `false`
for null values. This prevented the stale recovery job from recovering
channels that were already stuck before the fix was deployed.
Changing `isSyncStale` to return `true` for null/undefined allows the
stale job to properly reset stuck channels back to PENDING state.
neo773
29a2635164