objecter: don't serialize responses if there's no object name

objecter: don't serialize responses if there's no object name

This implicitly fixes an issue with the list_objects() being reentrant,
and triggers a lock dependency issue. The better solution would be to
have the callback context specify whether it's reentrant or not, but
will require a much bigger change.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>