[FIX] point_of_sale: prevent traceback while creating new pos category

When user create a pos category from 'restaurant.printer' model in
'pos_restaurant' module. it will give an error with the
 message -

'sequence item 0: expected str instance, bool found'

Steps to Produce:-

1. Install 'pos_restaurant' module
2. Go to 'point_of_sale' module
3. Go to 'Configuration' -> 'Order Printers'
4. Click on any printer or create new
5. Under 'Printed Product Categories' click 'Add a line'
6. Click 'New'

Trace-back will be generated.

Applying these changes will resolve this issue.

sentry - 4072172292

closes odoo/odoo#120519

Signed-off-by: Joseph Caburnay (jcb) <jcb@odoo.com>