[FIX] website_crm_partner_assign: Traceback in Partnership Analysis

Steps to reproduce the bug:

Go to CRM > Reporting > Partnership Analysis
Group by > Add Custom Group and select Invoice Account Date

Bug:

A traceback was raised

opw:2249977

closes odoo/odoo#50688

X-original-commit: 00fb1d7972057451dd0c58eb12b70fead5944aa6
Signed-off-by: Simon Goffin (sig) <sig@openerp.com>

addons/website_crm_partner_assign/report/crm_partner_report.py

@@ -50,7 +50,7 @@ class CrmPartnerReportAssign(models.Model):
                     p.team_id,
                     (SELECT count(id) FROM crm_lead WHERE partner_assigned_id=p.id) AS nbr_opportunities,
                     i.price_subtotal as turnover,
-                    i.invoice_date
+                    i.invoice_date as date
                 FROM
                     res_partner p
                     left join account_invoice_report i