One2Many display fails if field is null
From https://discuss.tryton.org/t/bom-tree-is-not-showing-the-required-input-quantities-for-a-output-product/1227
Form.display may call Widget.display with field = null but some widgets test only for undefined. This is no more true since r 5faa0ac6f80c.
But I still think we should use a better test to backport on older version and to be more consistent.