[FIX] mail: abstract getAvatarSource has default

Steps to reproduce:
- install sales
- create a new quotation

Previous behavior:
the avatar next to the author's name is empty in the mailthread

Current behavior:
avatar is displayed as expected

opw-2187231

closes odoo/odoo#44407

X-original-commit: 24ba9f99
Signed-off-by: mightyjol <jhk-odoo@users.noreply.github.com>