rgw: fix get_obj() with zero sized obj

Now that even zero sized objs have manifest a
test had to be modified.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit 4fbf9a75e23428fabb2a4b5f52e015a81685cdda)
rgw/rgw_rados.cc: use empty() instead of checking for size() > 0

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
(cherry picked from commit 22e0dbbd0e6b6a0fc3b49018d0a5d91ce54aca1b)