rgw: make rgw_sync_pipe_params::user optional

rgw: make rgw_sync_pipe_params::user optional

In rgw_sync_pipe_params, the mode can be either system or user.
When in system mode, no user is involved, but the current
implementation holds an empty rgw_user, which can cause confusion
in pipe_rules::find_basic_info_without_tags().

With this change, rgw_user is now optional, ensuring that when no
user is involved, it is explicitly nullopt rather than an empty object.

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