[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#41184

Issue: https://github.com/odoo/odoo/issues/41079


Signed-off-by: oco-odoo <oco-odoo@users.noreply.github.com>