net: lwm2m: remove special handling for message id 0

Change so that the caller of lwm2m_init_message is
responsible for generating a message id and remove message id generation
from lwm2m_init_message. Prevents generating a new id when the caller's
intent is to init a message with id 0.

Fixes #28283

Signed-off-by: Pascal Brogle <pascal.brogle@husqvarnagroup.com>