We should always ensure to assign something in a compute method
Since sms_template_id is not required, we have to check that it is set
before trying to render the template
Also make sure to have a default value in case you don't have a res_id
closesodoo/odoo#38546
X-original-commit: b96bb2a00737339dc53ff06167c1a0cd424ad717
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>