[FIX] web: fix error when getting image URL with default id

Linked to commit: https://github.com/odoo/odoo/commit/7eac23573c77415c84cb9c234c8063d4c68be425



Using false as default value instead of null will lead to the following
error if we try to get image URL of a record that is not the current one
and if no placeholder is available:
ValueError: invalid literal for int() with base 10: 'false'

opw-2516188

closes odoo/odoo#72610

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>