Comment Reply: Fix SSR for some cases

The cases are:
- The comment ID passed is invalid
- The comment doesn't have any parents
- There is no link to render

packages/block-library/src/post-comment-reply-link/index.php

@@ -24,13 +24,16 @@ function render_block_core_post_comment_reply_link( $attributes, $content, $bloc
 	}
 
 	$comment = get_comment( $block->context['commentId'] );
+	if ( empty( $comment ) ) {
+		return '';
+	}
 
 	$depth     = 1;
 	$max_depth = get_option( 'thread_comments_depth' );
 	$parent_id = $comment->comment_parent;
 
 	// Compute comment's depth iterating over its ancestors.
-	while ( 0 !== $parent_id ) {
+	while ( ! empty( $parent_id ) ) {
 		$depth++;
 		$parent_id = get_comment( $parent_id )->comment_parent;
 	}
@@ -43,6 +46,11 @@ function render_block_core_post_comment_reply_link( $attributes, $content, $bloc
 		$comment
 	);
 
+	// Render nothing if the generated reply link is empty.
+	if ( empty( $comment_reply_link ) ) {
+		return;
+	}
+
 	$classes = '';
 	if ( isset( $attributes['textAlign'] ) ) {
 		$classes .= 'has-text-align-' . $attributes['textAlign'];