Small change to allow non-shared articles with shared suppliers

We have one foodcoop that uses a single order for a wholesale supplier, but would like to add a handful of articles to the list not in the shared database. Currently, these handful of articles are deleted on sync.

This change allows foodcoops to leave or make the order number blank when they want to include manually maintained articles in a list of otherwise synchrosed articles.

Current behaviour when the order number is blank is not optimal - shared articles are found by order number, and while allowed, you can only have one (shared) article with a blank order number. Prone to errors.