When parsed, if the attachment has a basis quantity of zero,
it creates an error, because it is used in a division.
The condition that checks whether is computed
should be done only if this variable is none.

The code does not handle if `attachement_name.text` is None,
which can be caused by an orphan ID tag (e.i. "<cbc:ID />") ,
in the xml of the pdf.

opw-3470969

closes odoo/odoo#135472

X-original-commit: bfc83df7
Signed-off-by: William André (wan) <wan@odoo.com>
Signed-off-by: Laurent Smet (las) <las@odoo.com>