[FIX] mass_mailing: set bounced mail as bounced

Fix a mixup between `bounced_msg_id` that is sometimes tought as a list
of string or directly as a string.

Now `bounced_msg_id` is always a list or `False` if there is no bounce.

opw-2157793
closes #43084

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>