Force account move date when the value is empty.

Note that when the account move date is empty an exception:
constraint null value is raised during execution of action_move_create() method.

models/account_move.py

@@ -51,6 +51,9 @@
         """Override to set a transaction date from invoices.
         """
 
+        if not vals.get("date"):
+            vals["date"] = fields.Date.context_today(self)
+
         invoice = self.env.context.get("invoice")
         if invoice and isinstance(invoice, models.BaseModel):
             if not isinstance(vals, dict):