[IMP] hr: Add empty header for view inheritance

Some modules might want to add buttons in the form header of the employee public
(e.g. hr_appraisal).
This commit adds an empty header to easily allow view inheritance.

Note: this empty header is not displayed if no buttons are indide.

Task 2028794
PR odoo/enterprise#6495

closes odoo/odoo#39565

Related: odoo/enterprise#6495
Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>