#serialized_json for empty collection

Hi,

Behavior for serializer passed empty collection is to return nil

I think it would make more sense for the serializer to return `[]`. Ideally would work with empty Array and empty Relation.

```Ruby
##### Current behavior
UserSerializer.new([]).serialized_json
=> nil

##### Desired behavior(?)
UserSerializer.new(User.where.not(id: nil)).serialized_json
=> "[]"
```
