Some undefined indexes while debugging

Hi, while trying to debug another issue (redis purge failing) I ran into some undefined indexes. Here is a fix:

Replace lines 283 – 300 in admin/class-nginx-helper-admin.php by:

		$data['redis_enabled_by_constant'] = (
			defined( 'RT_WP_NGINX_HELPER_REDIS_HOSTNAME' ) &&
			defined( 'RT_WP_NGINX_HELPER_REDIS_PORT' ) &&
			defined( 'RT_WP_NGINX_HELPER_REDIS_PREFIX' )
		);

		if ( $data['redis_enabled_by_constant'] ) {
			$data['enable_purge']              = true;
			$data['cache_method']              = 'enable_redis';
			$data['redis_hostname']            = RT_WP_NGINX_HELPER_REDIS_HOSTNAME;
			$data['redis_port']                = RT_WP_NGINX_HELPER_REDIS_PORT;
			$data['redis_prefix']              = RT_WP_NGINX_HELPER_REDIS_PREFIX;
		}

		return $data;