[FIX] payment_stripe: allow payment for client without mail

Problem: If you have a customer assigned to an invoice/quotation
without an e-mail address and then share a payment link with
the customer. If the customer clicks on 'Pay', he won't be
redirected to Stripe's checkout page. He'll get instead 'Invalid
e-mail address: False' (see the image attached).

Explanation:
When there is no e-mail address defined, Odoo sends 'False' to
Stripe but Stripe sees it as the e-mail address which is indeed
Invalid. To solve the issue we replace False by None for the email
value of a customer without email address.

opw-3007866

closes odoo/odoo#102778

X-original-commit: beb580c2
Signed-off-by: Antoine Vandevenne (anv) <anv@odoo.com>
Signed-off-by: Fockedey Martin (mafo) <mafo@odoo.com>