libceph: add empty check to ceph_con_get_out_msg()

libceph: add empty check to ceph_con_get_out_msg()

This moves the list_empty() checks from the two callers (v1 and v2)
into the base messenger.c library.  Now the v1/v2 specializations do
not need to know about con->out_queue; that implementation detail is
now hidden behind the ceph_con_get_out_msg() function.

[ idryomov: instead of changing prepare_write_message() to return
  a bool, move ceph_con_get_out_msg() call out to arrive to the same
  pattern as in messenger_v2.c ]

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Reviewed-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>