Previously, if there was a sale_order_id but it was wrong,
like when the sale order was deleted for example, the function
didn't return a sale order, even with force_create=True.
It used to pass the first 'if' as sale_order_id had a value, so
no new sale order was created. However, as sometimes the id was
referring to a non-existant sale_order, a sale_order.exists() test
was used later, resulting in the function sometimes returning None,
even with force_create = True.
Proposed solution is to test the existence of the browse record with
the given id earlier, instead of testing the existence of the id itself.
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