Steps to reproduce the bug:
1: Invoice List -> Select few Invoices (With and Without Invoice Date filled in)
2: Action Confirm Draft Invoices

Bug: 
`TypeError: '<' not supported between instances of 'datetime.date' and 'bool'`

With this commit, we are using `fields.Date.context_today(self)` as default date if
`date_invoice` is not filled in.

Followup on  https://github.com/odoo/odoo/commit/6f319988ac15a366bcb370af870ec08cde75fb30


Fixes #73188

closes odoo/odoo#73529

Signed-off-by: William André (wan) <wan@odoo.com>