[FIX] website_sale: allow setting name for delivery

Fixup for d823033a

Portal user should not modify partner's name if an invoice for this partner is
already issued. However, delivery address contact may have empty name, while
eCommerce UI requires filling out the name. This blocks checkout process. Fix it
by allowing name changes, when it's not set.

opw-2981455

closes odoo/odoo#101791

X-original-commit: 781dbeac
Signed-off-by: William Braeckman (wbr) <wbr@odoo.com>