[FIX] sale: reinvoice only vendor bills
[FIX] sale: reinvoice only vendor bills

When validating a vendor bill, we want the product
with expense policy set to be reinvoice on the open
sales order matching the analytic account of the invoice
line. Applying the same mecanism for customer invoice
does not make sense, as we might confirm a SO and validate
its invoice, we don't want a new SO line to appear (avoid
loop).
Unfortunately, since a long time, this is possible in Odoo,
and really make no sense. We should only take negative
analytic lines (coming from vendor bills and expense, not
the customer invoice) into account for adding SO line.

This commit correct this problem to avoid having
many null SO line created on customer invoice validation.
All test cases are correctly written, but there was no
check to prevent this mecanism to be apply on customer
invoice.

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