Allow empty work center on work request
Currently the production work center is picked randomly from the available Work centers. This is not optimal when there are multiple work centers capable of doing a work as the user is not able to distinguis which work centers had been automatically chosen by the system (without a random criteria) and which ones had been chosen by a user.
There are some companies there is one person responible of chosing where each work should be performed, so it is not possible to have a list of pending works to assign for this user as they are always automatically picked by the system.
I think both cases will be better managed on Tryton if no Work Center is chosed when there is more than one capable of doing it so we force the user to decided on which one should be done. So in order to support this workflow we should make the work center non mandatory on request state.