Flags need to be set as 'optional' => true in CLI synopsys blocks

includes/classes/WPCLI/Command/GetSiteStats.php

@@ -74,6 +74,7 @@ public static function get_args(): array {
 					'name'        => 'clear-cache',
 					'description' => esc_html__( 'Clear the cache before retrieving the stats (can be intensive).', 'accessibility-checker' ),
 					'repeating'   => false,
+					'optional'    => true,
 				],
 			],
 		];