[FIX] base: print report without template

In case a report has no template defined, the fallback is on the first
body_parent that matched the xpath '//main'
xpath method returns a list

Fixes #27406

closes odoo/odoo#27630

odoo/addons/base/models/ir_actions_report.py

@@ -305,7 +305,7 @@ class IrActionsReport(models.Model):
         bodies = []
         res_ids = []
 
-        body_parent = root.xpath('//main')
+        body_parent = root.xpath('//main')[0]
         # Retrieve headers
         for node in root.xpath(match_klass.format('header')):
             body_parent = node.getparent()