Fixed for nested arrays given to SQL IN

This was reported first at https://github.com/rails/rails/issues/16580 but from my digging it seems it's a behaviour change in arel 6. Not sure if this should be fixed in rails (https://github.com/rails/rails/pull/16805) or in arel.

To be more clear:
```ruby
#arel 5
> User.arel_table[:id].in([[1],2,3]).to_sql
 => "users"."id" IN (1, 2, 3)

#arel 6
> User.arel_table[:id].in([[1],2,3]).to_sql
 "users"."id" IN (NULL, 2, 3)
```

/cc @tenderlove