Fix broken in/nin matchables where attribute is nil

```ruby
class ParentDoc
  include Mongoid::Document

  embeds_many :child_docs
end

class ChildDoc
  include Mongoid::Document

  field :attribute

  embedded_in :parent_doc
end

ParentDoc.create!(child_docs: [ { attribute: nil } ])

ParentDoc.in(:'child_docs.attribute' => [nil]).count => 1
ParentDoc.in(:'child_docs.attribute' => [nil]).first.child_docs.in(:attribute => [nil]).count => 0
```