Do not render if no fallback can be found for the Nav block (#36849)

packages/block-library/src/navigation/index.php

@@ -187,14 +187,20 @@ function( $block ) {
  * @return array the array of blocks to be used as a fallback.
  */
 function block_core_navigation_get_fallback_blocks() {
-	// Default to a list of Pages.
-	$fallback_blocks = array(
+	$page_list_fallback = array(
 		array(
 			'blockName' => 'core/page-list',
 			'attrs'     => array(),
 		),
 	);
 
+	$registry = WP_Block_Type_Registry::get_instance();
+
+	// If `core/page-list` is not registered then return empty blocks.
+	$fallback_blocks = $registry->is_registered( 'core/page-list' ) ? $page_list_fallback : array();
+
+	// Default to a list of Pages.
+
 	$navigation_post = block_core_navigation_get_first_non_empty_navigation();
 
 	// Prefer using the first non-empty Navigation as fallback if available.
@@ -298,6 +304,11 @@ function render_block_core_navigation( $attributes, $content, $block ) {
 
 		$fallback_blocks = block_core_navigation_get_fallback_blocks();
 
+		// May be empty if core/navigation or core/page list are not registered.
+		if ( empty( $fallback_blocks ) ) {
+			return '';
+		}
+
 		$inner_blocks = new WP_Block_List( $fallback_blocks, $attributes );
 	}