With no differential pressure, upstream and downstream sealing force on floating ball is the same.
Definitely cannot be zero force. Floating ball seats need to support the weight of the ball at a minimum. Then extra compression is added to account for any plastic deformation of the seats, manufacturing dimensional tolerances, and ensure low-pressure sealing capability.
FEA can be used to get a close prediction of the seat force when valve is first assembled. But you should verify your computer models and material properties with tests.