Support empty lists in IN() clause

Currently the following LINQ statement throws an exception if keys is empty:
`table.Where(item => keys.Contains(item.pk)).Execute()`

However, empty lists are supported by Cassandra: the following CQL statement is valid:
`SELECT * FROM "x_t" WHERE "x_pk" IN ()`

I propose to remove this exception, and leave it up to the user whether he/she wants to avoid a round-trip to the database if the key list is empty.