python: domain: models: as_dict() should also exclude empty list fields

python: domain: models: as_dict() should also exclude empty list fields

Empty list fields happen if many=True is used on the field. This means that the field is automatically initialised as an empty list, so this can only ever be sa list or None.

The side-effect of this was that it appears in as_dict() when it shouldn't, because the field isn't populated. This fixes it.

Signed-off-by: Rob van der Linde <rob@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>