In some cases, empty messages can be displayed in the portal chatter.

For instance, if a customer sends a message with no body and just
an attachment, and the attachment then gets deleted, it will still
be displayed as an empty message in the portal chatter.

To solve this, this commit filters out empty messages from the portal chatter.

Task-2833919

closes odoo/odoo#89742

Signed-off-by: Warnon Aurélien (awa) <awa@odoo.com>