Make current_state work with an empty string

Not sure it is a responsibility of gem. But maybe it will be useful.
I have an ActiveRecord model, let's say `Seller` with existed records. I decided to add a state_machine and add a column `state`:

```ruby
class Seller < ActiveRecord::Base
  include ActiveModel::Transitions

  state_machine attribute_name: :status, initial: :not_reviewed do
    state :not_reviewed
    state :verified

    event :verify do
      transitions to: :verified, from: [:not_reviewed]
    end
  end
end
```

I was surprised when I got:

```ruby
seller = Seller.last # => { id: 1, status: '' }
seller.current_state # => :not_reviewed
seller.current_state # => :""
```

To fix it I'm suggesting to fix `current_state` method