Only hide empty values if compact mode is enabled

[fuegas]

If a value is an empty string, it currently is hidden regardless of the setting compact_show_view.

I think an empty string should only be hidden if compact mode is enabled. This PR enables that behaviour.

[mshibuya]

Could you write a spec for this issue? Best place will be:
https://github.com/sferik/rails_admin/blob/master/spec/integration/actions/show_spec.rb

[fuegas]

@mshibuya : Tests have been added. When you run these without the parentheses in the view the test shows blank fields will fail:

  2) Show action when compact_show_view is disabled shows blank fields
     Failure/Error: is_expected.to have_css('.logo_url_field')
       expected #has_css?(".logo_url_field") to return true, got false
     # ./spec/integration/actions/show_spec.rb:95:in `block (3 levels) in <top (required)>'

[mshibuya]

Perfect, thanks!