Make sure get_m2m works when no rows match

[dantownsend]

If you use get_m2m to get the related objects:

https://piccolo-orm.readthedocs.io/en/latest/piccolo/schema/m2m.html#get-m2m

It will raise a ValueError if there are no matching objects, because under the hood Piccolo uses the is_in clause which requires at least one value.

We should handle this edge case and return an empty list instead.

[dantownsend]

Fixed by #1089