Check if cart is NOT empty in Woocommerce 3
add_action( 'wp_footer', 'vazio' ); function vazio() { if (sizeof( WC()->cart->get_cart() ) > 0 ) { // do something } }This will check to see if there are items in the cart. You can add an else statement or check for equivalence as needed.
In new woocommerce 2.1+ : WC()->cart->cart_contents_count to check cart content count
add_action("template_redirect", 'redirection_function');function redirection_function(){ global $woocommerce; if( is_cart() && WC()->cart->cart_contents_count == 0){ wp_safe_redirect( get_permalink( woocommerce_get_page_id( 'shop' ) ) ); }}To check the cart content you can also use this code:
global $woocommerce;if ( $woocommerce->cart->cart_contents_count != 0 ) { // cart has content} else { // cart is empty}
For ajax add to cart you will also want to add this to your JS file:
$('body').on( 'added_to_cart', function(){ if( ! $(this).hasClass('has_items') ) { // do something }});