[IMP] hr: Make timezone required on saved record

`tz` field is technical field related to `resource_id`, which is required field on that model.

However it is possible to try to write a void value, which leads to an issue as it is technically required.
With this commit we make the tz field required at write.

closes odoo/odoo#45557

Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>

addons/hr/views/hr_employee_views.xml

@@ -116,7 +116,8 @@
                                         </group>
                                         <group string="Schedule" groups="base.group_no_one">
                                             <field name="resource_calendar_id" required="1"/>
-                                            <field name="tz"/>
+                                            <field name="id" invisible="1"/>
+                                            <field name="tz" attrs="{'required': [('id', '!=', False)]}"/>
                                         </group>
                                     </div>
                                 </div>