net: lwm2m: Bypass send_queue when sending empty Ack

In case we want to immediately send empty Ack to server,
we should bypass all send queues.

This is required when we try to send Ack from callbacks
that happen from socket-loop context. On those cases
the Ack would have not been send because the callback
might be blocking the socket-loop while processing
a request (like write callbacks).

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>