[FIX] base: Preserve loosing embedded files when printing 1 attachment

With the reload from attachment ticked (attachment_use) when dealing with
a single pdf, the attachment's content must be returned without calling _merge_pdfs
in order to preserve the embedded files. Since PyPDF2 doesn't support completely
the embedded files, there are simply dropped with creating a new pdf in _merge_pdfs even
with a single attachement.

closes odoo/odoo#41246

Issue: https://github.com/odoo/odoo/issues/41079
X-original-commit: a8f96bc7f35d047999cd16b7dd03bcddd3c32263
Signed-off-by: Laurent Smet <smetl@users.noreply.github.com>

odoo/addons/base/models/ir_actions_report.py

@@ -559,8 +559,10 @@ class IrActionsReport(models.Model):
                     pass
 
         # Check special case having only one record with existing attachment.
+        # In that case, return directly the attachment content.
+        # In that way, we also ensure the embedded files are well preserved.
         if len(save_in_attachment) == 1 and not pdf_content:
-            return self._merge_pdfs(list(save_in_attachment.values()))
+            return list(save_in_attachment.values())[0].getvalue()
 
         # Create a list of streams representing all sub-reports part of the final result
         # in order to append the existing attachments and the potentially modified sub-reports