do not check every 5 seconds files with infinite timeout (= 0)

do not check every 5 seconds files with infinite timeout (= 0)

we want to schedule a new sync attempt if a file is likely to have been
unlocked due to timeout expiring on server

but if the timeout is null (equal to 0), there will be no timeout on
server and no automated unlocking, so do not schedule a sync every 5
seconds to check for it

Signed-off-by: Matthieu Gallien <matthieu.gallien@nextcloud.com>