You should use 'placeholder
' instead of 'empty_value
'
->add('playerPosition', 'choice', array(
'choices' => array('Spelverdeler' => 'Spelverdeler', 'Receptie/Hoek' => 'Receptie/Hoek', 'Libero' => 'Libero', 'Midden' => 'Midden', 'Opposite' => 'Opposite', 'All-round' => 'All-round'),
'label' => 'Spelerspositie*',
'placeholder' => 'Kies uw positie.',
'required' => true
))