in_tail: Fix error on setting page for binary file

[kou]

How to reproduce:

• Create a binary file by the following command:

  "ruby -e 'puts "\u3042"' > /tmp/xxx"
  

• Open /daemon/setting/in_tail

• Select "/tmp/xxx"

• Error page is rendered

• log/production.log says:

  ActionView::Template::Error (undefined method `join' for nil:NilClass):
      12:     = f.text_field :path, class: "form-control", disabled: true
      13:   = render partial: "shared/vue/in_tail_format", locals: { file: f.object.path, formats: @setting.known_formats, initialSelected: f.object.format || @setting.guess_format }
      14:
      15:   %pre= file_tail(@setting.path).join("\n")
      16:
      17:   %p
      18:     = f.submit t('terms.next'), class: "btn btn-lg btn-primary pull-right"
    app/views/fluentd/settings/in_tail/after_file_choose.html.haml:15:in `block in _app_views_fluentd_settings_in_tail_after_file_choose_html_haml___1713159989942914940_40548520'
  

[uu59]

I can see the same issue as given steps, and confirm as fixed in this patch.
Thank you for your contribution!

[kou]

Thanks for merging it!
It seems that this product is useful...!