iproto: add an empty body to the unprepare response

Absence of the body in the unprepare response forces users to perform
additional checks to avoid errors. Adding an empty body fixes this problem.

Closes #4769

Reviewed-by: Vladislav Shpilevoy <v.shpilevoy@tarantool.org>
Reviewed-by: Nikita Pettik <korablev@tarantool.org>