[FIX] point_of_sale: make the email button behave as the invoice

This commit makes the email button to behave consistently with the
invoice button. Basically, if there is no selected customer when
invoicing, the UI should direct the user to select a customer. This
behaviour is implemented with the email button with special check on the
validity of the customer email.

closes odoo/odoo#36681

Signed-off-by: pimodoo <pimodoo@users.noreply.github.com>