Jetpack: Fix REST API warnings from Comments endpoint (#39102)

There are two bugs involved here:

* The Subscriptions block is trying to hide comments by returning
  empty-string from the 'get_comment' filter, but that filter must
  return a WP_Comment object. Not even null would be right.
* The REST API endpoint isn't checking for if `get_comment()` on the
  parent comment ID returns null.

This fixes the second; I don't know what would be correct to fix the
first.

projects/plugins/jetpack/changelog/fix-jetpack-comment-endpoint-warnings

@@ -0,0 +1,4 @@
+Significance: patch
+Type: bugfix
+
+REST API: Avoid PHP warnings in comment endpoint when the parent comment does not exist.

projects/plugins/jetpack/json-endpoints/class.wpcom-json-api-comment-endpoint.php

@@ -185,8 +185,8 @@ public function get_comment( $comment_id, $context ) {
 					$response[ $key ] = (string) $status;
 					break;
 				case 'parent': // May be object or false.
-					if ( $comment->comment_parent ) {
-						$parent           = get_comment( $comment->comment_parent );
+					$parent = $comment->comment_parent ? get_comment( $comment->comment_parent ) : null;
+					if ( $parent ) {
 						$response[ $key ] = (object) array(
 							'ID'   => (int) $parent->comment_ID,
 							'type' => (string) ( $parent->comment_type ? $parent->comment_type : 'comment' ),