Always return a date for tax date of TaxableMixin

Closes #14486

modules/account_invoice/invoice.py

@@ -2752,6 +2752,8 @@ class InvoiceLine(sequence_ordered(), ModelSQL, ModelView, TaxableMixin):
             return self.taxes_date
         elif hasattr(self, 'invoice') and hasattr(self.invoice, 'tax_date'):
             return self.invoice.tax_date
+        else:
+            return super().tax_date
 
     def _get_tax_context(self):
         if self.invoice:

modules/sale_point/sale.py

@@ -446,7 +446,7 @@ class POSSale(Workflow, ModelSQL, ModelView, TaxableMixin, ChatMixin):
 
     @property
     def tax_date(self):
-        return self.date
+        return self.date or super().tax_date
 
 
 class POSSaleLine(ModelSQL, ModelView, TaxableMixin):
@@ -719,7 +719,7 @@ class POSSaleLine(ModelSQL, ModelView, TaxableMixin):
 
     @property
     def tax_date(self):
-        return self.sale.date
+        return self.sale.date or super().tax_date
 
 
 class POSCashSession(Workflow, ModelSQL, ModelView):