[FIX] base_setup: don't crash if template is empty
[FIX] base_setup: don't crash if template is empty

Before this commit:

* Activate debug mode.
* Go to general settings > Document Template.
* Change the Template selection to the empty value.
* Click on "Edit Header".

Result: Traceback.

This happens because we try to concatenate the value of the template
field to a string, and since it's empty (False) boolean and str can't be
implicitly concatenated, this of course raises an uncaught error.

After this commit:

* We check if a template has been selected before performing the
concatenation.

Closes #22736
Fixes #22622
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo