Allow empty string be a thousands separator

If you pass empty string as thousands separator every price greater than two symbols would be invalid. For example:

```yml
# config/locales/en.yml
en:
  number:
    currency:
      format:
        delimiter: ''
```

```ruby
product.price = '10.00'
product.valid? #=> false
```

This may be fixed be replacing delimiter by `nil` but I think that in sake of convenience it's better to check that case on the `money-rails` side.