[FIX] hr_work_entry_contract: traceback issue during work entry regeneration

In this commit,
1. Adding a safeguard to ensure date_from is not empty before calculating the
time delta in the wizard.date_to.
2. Both classes disabled="1" and disabled="true" are not working so we add
a disabled class on the button. I don't know why disabled="1" is not working.

task-3328601

closes odoo/odoo#123148

X-original-commit: 97b8d59a88e78afbeae1dc6447a3d9629635524f
Signed-off-by: Kevin Baptiste <kba@odoo.com>

addons/hr_work_entry_contract/wizard/hr_work_entry_regeneration_wizard.py

@@ -26,7 +26,7 @@ class HrWorkEntryRegenerationWizard(models.TransientModel):
     @api.depends('date_from')
     def _compute_date_to(self):
         for wizard in self:
-            wizard.date_to = wizard.date_from + relativedelta(months=+1, day=1, days=-1)
+            wizard.date_to = wizard.date_from and wizard.date_from + relativedelta(months=+1, day=1, days=-1)
 
     @api.depends('employee_ids')
     def _compute_earliest_available_date(self):

addons/hr_work_entry_contract/wizard/hr_work_entry_regeneration_wizard_views.xml

@@ -55,8 +55,7 @@
                             attrs="{'invisible': ['|', ('search_criteria_completed', '=', False), ('valid', '=', False)]}"/>
                     <button name="regenerate_work_entries_disabled"
                             string="Regenerate Work Entries"
-                            class="btn btn-primary"
-                            disabled="1"
+                            class="btn btn-primary disabled"
                             attrs="{'invisible': [('search_criteria_completed', '=', True), ('valid', '=', True)]}"/>
                     <button name="cancel_button" string="Cancel" class="btn-secondary" special="cancel"/>
                 </footer>