[FIX] sale_coupon: send mail template to partner of SO

... instead of public user

Once sending the template email Coupon: Send by Email.
Odoo was sending to the partner linked to the sale.coupon

However in the case of the program having the parameters:
apply on next order & apply automatically.
The partner_id.id is the public user.

This commit is modifying the value of the template to send
the email to the partner of the SO instead of the partner
of the current coupon.

opw-2255674

closes odoo/odoo#51570

X-original-commit: f7e0c2d1568b222d510d3a10fba1116fabfff604
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/coupon/data/coupon_email_data.xml

@@ -6,7 +6,7 @@
          <field name="model_id" ref="coupon.model_coupon_coupon"/>
          <field name="subject">Your reward coupon from ${object.program_id.company_id.name} </field>
          <field name="email_from">${object.program_id.company_id.email | safe}</field>
-         <field name="partner_to">${object.partner_id.id}</field>
+         <field name="partner_to">${object.order_id.partner_id.id or object.partner_id.id}</field>
          <field name="body_html" type="html">
 <table border="0" cellpadding="0" cellspacing="0" style="width:100%; margin:0px auto;"><tbody>
     <tr><td valign="top" style="text-align: center; font-size: 14px;">