Describe the bug

When loading the post editing screen, the <PostAuthor/> component triggers a REST API request (via getAuthors()) to fetch the authors to list to populate its filterable list. If the user isn't planning on changing the post author, this is an unnecessary REST API request and a waste of resources.

This request should be performed on-demand, for example when the field gains focus.

To reproduce

1. Go to the post editing screen on a site with several users
2. Observe that a request to wp-json/wp/v2/users is performed
3. Don't change the post author
4. Wonder why that request was needed

Editor version (please complete the following information):

• WordPress version: 5.6 beta