WorkOrders can only be approved if they have a client

Only let users approve a workorder if a client is given. Without that, since
the editor is a disguised wizard, one could start working without the client
approval. That would generate some tracebacks when trying to get the price by
the client's category.

Change-Id: Ieb730e3e07547d61c107d709fec5fdae47f8aacd
Merge "WorkOrders can only be approved if they have a client"