Bugfix: UI for preview bug related to single language & non variant content

[madsrasmussen]

How to reproduce:

• Enable the delivery API - set Umbraco:CMS:DeliveryApi:Enabled to true in appsettings.json
• Use the code from a new PR with a partial UI fix Ensure additional preview URLs work on non-variant content #14995 or use the examples from the docs: https://docs.umbraco.com/umbraco-cms/reference/content-delivery-api/additional-preview-environments-support#configuring-custom-preview-urls
• Have a single language & non-variant content
• Make the content type vary by culture
• Visit the content node & you will see that the button group for previewing additional Preview URLs is not there. We just have the regular "Save and preview" button.
  Another issue related to this (continue from the preview steps...):
• Add a new language
• In the Content section - try to create a new content node in the new language
• Without inputting any info, the default buttons we get are "Save and preview", "Save" and "Save and publish" - we are missing the button group here as well

What to test:

• Make sure that the additional preview URLs are shown when a document is published.

[elit0451]

Works as expected 🥳 Merging