[FIX] mrp: handle the case of empty routings

Before, when the routing of a production order was empty (no operations),
it got into an inconsistent state (you click on "plan" and nothing happens).

Empty routings were used before to do subcontracting or 2-steps inventeory
(we did not set any operations on the routing but we set a location).

Here, we change the related routing field on the production order into a
calculated field which becomes empty when the routing has no operations.
This way, the "produce" button is displayed (as for production orders
without routing) and working/

This is a temporary patch to workaround the issuein a stable release.
In master, we might remove the location on routing and place it on the
workcenters and put a constraint that does not allow an empty routing.

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