rgw: avoid shard recreation during bilog trimming if it doesn't exist

rgw: avoid shard recreation during bilog trimming if it doesn't exist

When trimming bilogs, avoid recreating shards that don't exist. Instead,
calling assert_exists() will return ENOENT, and the result will be logged
in handle_result() without unnecessary shard creation.

Signed-off-by: Seena Fallah <seenafallah@gmail.com>