[FIX] website: fix background video widget on mobile

Issue

    - Download the customer db: stargue-inc
    - Go on the website in mobile mode

    Traceback & the second item in the carousel is not displayed.

Cause

    Traceback: we use `this.$iframe` without checking if it's defined
    Second item not displayed: the first video promise is overriden
    by the second video promise.

Solution

    Check if this.$iframe is defined and resolve the old promise
    instead of overriding it.

OPW-2152690

closes odoo/odoo#42139

X-original-commit: f5f569b4
Signed-off-by: Jason Van Malder <jvm-odoo@users.noreply.github.com>