Comments: Fix fatal error when get_comment_author() receives an object with no comment_id.

[58335] introduced `(string)` type casting of the passed in `$comment_id` value. If `$comment_id` is a scalar, it works as expected. But if it's an `object`, the following fatal error is thrown:

{{{
Object of class WP_Comment could not be converted to string
}}}

This fatal error happens when the incoming `$comment_id` is an instance of `WP_Comment` (or any object) without a `comment_ID` (empty). 

This changeset adds tests to demonstrate the fatal error and validate the fix.

It fixes the fatal error by restructuring the ternary checks into an `if/elseif/else` structure for the 3 paths:

- When `$comment->comment_ID` is not empty, then it uses the property.
- When `$comment_id` is scalar, then it type casts it to a `string`.
- Else, the default is an empty `string`.

Follow-up to [58335], [41127], [52818].

Props ambrosiawt, hellofromTonya, jorbin, mukesh27, SergeyBiryukov.
Fixes #61681.
Built from https://develop.svn.wordpress.org/trunk@58755


git-svn-id: http://core.svn.wordpress.org/trunk@58157 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/comment-template.php

@@ -24,7 +24,13 @@
 function get_comment_author( $comment_id = 0 ) {
 	$comment = get_comment( $comment_id );
 
-	$comment_id = ! empty( $comment->comment_ID ) ? $comment->comment_ID : (string) $comment_id;
+	if ( ! empty( $comment->comment_ID ) ) {
+		$comment_id = $comment->comment_ID;
+	} elseif ( is_scalar( $comment_id ) ) {
+		$comment_id = (string) $comment_id;
+	} else {
+		$comment_id = '';
+	}
 
 	if ( empty( $comment->comment_author ) ) {
 		$user = ! empty( $comment->user_id ) ? get_userdata( $comment->user_id ) : false;

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.7-alpha-58754';
+$wp_version = '6.7-alpha-58755';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.