[FIX] pos_restaurant: get table draft orders without employee

Before this commit: In the PoS Restaurant, if the employee is not
installed, the orders wouldn't contain employee_id, and UI couldn't get
table draft orders.

Steps to reproduce the first issue:

	- Install the' Point of Sale' module
	- Make sure Employees (hr) isn't installed
	- Create a PoS with "Is a Bar/Restaurant" enabled
	- Open a PoS session in the incognito window
	- Add the product to the orders for different tables
	- Close the incognito window and open it again

	It couldn't get table draft orders from the back-end.

Solution

	It must check that "employee_id" exist in the dictionary.

opw-2856653

closes odoo/odoo#92093

Signed-off-by: Trinh Jacky (trj) <trj@odoo.com>