unwrap links without an href

What does this MR do?

Fixes #27645 (closed) by unwrapping links with a blank href, which can be added by outlook.

Are there points in the code the reviewer needs to double check?

I believe it makes sense for us to do this within Gitlab, since it's specific to email. For the rich-text example, Html2Text can do a better job of not creating empty links, but for the client's later example, the entirety of the content is wrapped in a link, so there's not much to be done generically.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?