rgw/datalog: also handle errors properly for fifo trim

rgw/datalog: also handle errors properly for fifo trim

RGWDataChangesFIFO::trim: handle ENODATA from empty FIFO: fifo::FIFO::trim()
throws ENODATA (no_message_available) when the FIFO has no parts yet.

This manifests on clusters with rgw_default_data_log_backing=fifo when
trim_entries is called on a shard before any entries have been pushed.

Signed-off-by: Oguzhan Ozmen <oozmen@bloomberg.net>