Comments: Use comment instead of an empty string for the comment_type DB field value in comments table.

This is the first step to bring support for custom comment types into WordPress.

Add a scheduled upgrade routine to update the type value for existing comments, in batches of 100 at a time.

Props imath, aaroncampbell, jeremyfelt, dshanske.
Fixes #49236.

git-svn-id: https://develop.svn.wordpress.org/trunk@47597 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-comment.php

@@ -114,9 +114,10 @@ final class WP_Comment {
 	 * Comment type.
 	 *
 	 * @since 4.4.0
+	 * @since 5.5.0 Default value changed to `comment`.
 	 * @var string
 	 */
-	public $comment_type = '';
+	public $comment_type = 'comment';
 
 	/**
 	 * Parent comment ID.