[FIX] hr_timesheet: Hide timesheets on tasks w/o project

Since it is not possible to create a timesheet entry on a task without a project_id,
the timesheets page on the task form view will be hidden on such tasks.
This prevents the user from trying to create a timesheet entry on such a task and
receive a not so transparent message like: mandatory field is not set: [object with reference: account_id - account.id]

addons/hr_timesheet/project_timesheet_view.xml

@@ -75,7 +75,7 @@
                     <field name="subtask_project_id" invisible="1" />
                 </field>
                 <xpath expr="//notebook/page[@name='description_page']" position="after">
-                    <page string="Timesheets">
+                    <page string="Timesheets" attrs="{'invisible': [('project_id', '=', False)]}">
                     <field name="timesheet_ids" context="{'default_project_id': project_id}">
                         <tree editable="bottom" string="Timesheet Activities" default_order="date">
                             <field name="date"/>