When searching if a many2one property field is not set, there may be less
results since only the ones with a reference set to NULL are returned.
We should also get those not in the table.
This commit change this case so instead of returning ['id', 'in', {matching non-set ids}],
the ['id', 'not in', {matching set ids}] is returned.
e.g: if (1, 3, 8) are set, (5, 9) are not set. ['id', 'not in', (1, 3, 8)] would
be returned instead of ['id', 'in', (5, 9)] which might not select all non-set
property fields.
closes#6044
opw-631057
Nicolas Lempereur
Martin Trigaux