Allow for default_locale to be nil

[jtomaszewski]

Feature request:

1. If default_locale is false, (or we didnt found a default_locale in `available_locales), then:
   • if object has already some translations, just set as default the first one found
   • if object has no translations (f.e. when its a new_record?), it should show only available_locales list (we need to explicitly add first translation, to make the translatable_inputs) visible

Changes would be need to be done propably in https://github.com/fabn/activeadmin-globalize/blob/develop/lib/active_admin/globalize/form_builder_extension.rb .

I might to do a PR if interested.