Editor: Return previewLink from post link only if available

[aduth]

Fixes #12677

This pull request seeks to resolve an error which occurs when autosaving a post whose post type is registered using 'publicly_queryable' => false.

An in-depth explanation of the issue can be found at #12677 (comment) .

Testing instructions:

Verify using the following example post type definition that autosave occurs without error:

<?php

/**
 * Plugin Name: Demo CPT
 */
add_action( 'init', function() {
	register_post_type( 'book', [
		'label' => 'Book',
		'show_in_rest' => true,
		'public' => true,
		'publicly_queryable' => false,
		'supports' => [ 'title', 'editor', 'revisions' ],
	] );
} );

[jorgefilipecosta]

LGTM 👍 It fixed the problem in my tests.