[FIX] web: kanban: conditions on relational fields

This commit aims to correctly fill in the raw_value of each
field in the qweb_context use to render the kanban templates.
For relational fields, the absence of value should be false,
instead of an empty array. Moreover, the Many2One case is
different from the x2Many cases as in the first case the
datapoint is a record, and it the last ones it is a list.