ps-dont-set-content-to-nil-while-aggregating
Run COALESCE on log parts aggregate
If there're no parts for a given log array_to_string will return NULL and in consequence concatenation will also return NULL (because as it turns out `'' || NULL` returns NULL). This theoretically shouldn't happen, because aggregate service gets only logs that contain parts, but if a restart is performed while the aggregate service runs, it can potentially clear log parts for a log that is already scheduled for aggregation.