Fix TOC nesting: initialize stack properly when first heading isn't h2

Author: ajaydsouza

includes/frontend/class-toc.php

@@ -186,9 +186,11 @@ private static function build_toc_html( array $headings, array $args ): string {
 				}
 				if ( ! empty( $stack ) && end( $stack ) === $level ) {
 					$output .= '</li>';
-				} else {
+				} elseif ( ! empty( $stack ) ) {
 					$output .= '<ul>';
 					$stack[] = $level;
+				} else {
+					$stack[] = $level;
 				}
 			}