[FIX] delivery: prevent trace back while partner has no zip

In delivery address, `country name` and in shipping method, `country name`
will be same and no zip in delivery address.  While creating new sale order
along with product and clicking on "Add shipping",  error will be generated.

Steps to Reproduce
-Install `sale_management' and 'delivery' modules
-Go to the settings and enable the 'Customer Address'.
-Go to the settings and enable 'Shipping Methods' and configure it.
-Select a shipping method and  go to the 'Destination Availability' tab and
 set to the 'Countries' and set to 'Zip Prefixes'.
-Create a new customer and add the delivery address of the customer in
 res.partner.
-The delivery address and shipping method of the 'Countries" or 'Country'
 name should be the same.
-Set the  'Zip Prefixes'
-Set the delivery address of zip code null.
-Create a new quotation and add to the customer and delivery address
-Add to the product in the sale order line.
-Click on the 'Add Shipping' or 'Update Shipping Cost' button.

A trace back will be generated.

Applying this commit will resolve this issue.

sentry-4147077852

closes odoo/odoo#120802

Signed-off-by: Tiffany Chang <tic@odoo.com>