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.

(cherry picked from commit e91e2e99c4104b2294b01d786888092fdc473445)

Change-Id: Id4df2bbe908a38638c4cec5a817bc1e62eb5d1fa