[FIX] base: Preserve loosing embedded files when printing 1 attachment
[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 <1328e3a5817ff065354bd0f7f4073cbb6dcc7eca@users.noreply.github.com>

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