Create filter and improve output validity test

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

@@ -187,6 +187,7 @@ 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(
 		array(
@@ -206,7 +207,7 @@ function block_core_navigation_get_fallback_blocks() {
 		$fallback_blocks = ! empty( $maybe_fallback ) ? $maybe_fallback : $fallback_blocks;
 	}
 
-	return $fallback_blocks;
+	return apply_filters( 'block_core_navigation_render_fallback', $fallback_blocks );
 }
 
 /**
@@ -298,7 +299,13 @@ function render_block_core_navigation( $attributes, $content, $block ) {
 
 		$fallback_blocks = block_core_navigation_get_fallback_blocks();
 
+		// Fallback my have been filtered so do basic test for validity.
+		if ( empty( $fallback_blocks ) || ! is_array( $fallback_blocks ) ) {
+			return '';
+		}
+
 		$inner_blocks = new WP_Block_List( $fallback_blocks, $attributes );
+
 	}
 
 	// Restore legacy classnames for submenu positioning.