Automatically make sole, required terms the selected default

Issue Summary

Here's a simple little feature that could live in the taxonomy defaults module.

If there is only one taxonomy term, and it is required, select by default.

There is an issue to add this to Drupal core, taxonomy term selected by default when there is only one. Taxonomy defaults of course at least makes this possible, to select any taxonomy term by default, but this could also be provided as a nice automatic enhancement of the user experience.

^{ben, agaric}