[FIX] mrp: prevent the unplan of a single work order

Steps to reproduce the bug:
Go to a work order that is ready or waiting,  try manually changing the scheduled start or end date > save

Problem:
Traceback is triggered because when we modify the date, we check if the start date is before the end date. But we cannot compare a False with a date

opw-2768054

closes odoo/odoo#85991

X-original-commit: 1768a072fd40d14dc8c402c3ccad9a6733460c03
Signed-off-by: William Henrotin (whe) <whe@odoo.com>
Signed-off-by: Djamel Touati (otd) <otd@odoo.com>

addons/mrp/i18n/mrp.pot

@@ -2184,6 +2184,14 @@ msgstr ""
 msgid "It has already been unblocked."
 msgstr ""
 
+#. module: mrp
+#: code:addons/mrp/models/mrp_workorder.py:0
+#, python-format
+msgid ""
+"It is not possible to unplan one single Work Order. You should unplan the "
+"Manufacturing Order instead in order to unplan all the linked operations."
+msgstr ""
+
 #. module: mrp
 #: model:ir.model.fields,field_description:mrp.field_mrp_production__is_planned
 msgid "Its Operations are Planned"

addons/mrp/models/mrp_workorder.py

@@ -252,6 +252,9 @@ class MrpWorkorder(models.Model):
             workorder.date_planned_finished = workorder.leave_id.date_to
 
     def _set_dates_planned(self):
+        if self.leave_id and (not self[0].date_planned_start or not self[0].date_planned_finished):
+            raise UserError(_("It is not possible to unplan one single Work Order. "
+                              "You should unplan the Manufacturing Order instead in order to unplan all the linked operations."))
         date_from = self[0].date_planned_start
         date_to = self[0].date_planned_finished
         self.mapped('leave_id').sudo().write({