replaced null with empty string because behind the scene it uses strp…

…os() and str_replace() where a null value is deprecated. (#443)

Co-authored-by: Aashish <aashish@omise.co>

includes/class-omise-admin.php

@@ -64,7 +64,7 @@ public function wordpress_hook_admin_menu()
 		public function wordpress_hook_card_form_customization()
 		{
 			add_submenu_page(
-				null,
+				"",
 				__('Custom card form customization', 'omise'),
 				Omise_Page_Card_From_Customization::PAGE_NAME,
 				'manage_options',