[FIX] purchase: wrong condition in mail template

Before checking the first element of
'partner_id.child_ids', rather check that
the partner and not the object has children,
to avoid crashes when the partner of the PO
is a company.
opw-1826359

addons/purchase/data/mail_template_data.xml

@@ -15,7 +15,7 @@
             <field name="lang">${object.partner_id.lang}</field>
             <field name="body_html"><![CDATA[
 <p>Dear
-% if object.partner_id.is_company and object.child_ids:
+% if object.partner_id.is_company and object.partner_id.child_ids:
     ${object.partner_id.child_ids[0].name}
 % else :
     ${object.partner_id.name}