By default, there is an automatic redirect from the WooCommerce checkout page to the (empty) cart page when there are no products in the cart.
Try accessing the checkout page (link in footer) at this sample store and you will see the redirect.
There are uncommon situations where you may want to disable this redirect and show the user the checkout page anyway. You can do this by adding the following code to your functions.php file:
add_filter( 'woocommerce_checkout_redirect_empty_cart', '__return_false' );
add_filter( 'woocommerce_checkout_update_order_review_expired', '__return_false' );