[FIX] mail: do not compute chat windows in DiscussPublicView

__Current behavior before PR:__
When someone joins a call in the discuss public view and receives a message in the chatter, the part of the screen supposed to display the video streams of the participants goes black.

This happens because it is trying to open a chat window in the bottom right as it is done usually. However since we are in the public view it removes it instantly together with the participants cards.

__Description of the fix:__
If we are in the discuss public view, it is useless to make chat windows. This will prevent opening of chat windows.

__Steps to reproduce the issue:__
1. Open the discuss app and start a meeting.
2. Copy the Invitation Link
3. Open a new private window to join the call with a guest user.
4. Then go back to the non-guest user page and send a message in the chatter.
5. Notice that the participants cards vanished on the private window page.

opw-3229747

closes odoo/odoo#119359

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>