Put this in your Functions.php file of your active theme.
// Remove default link around product entries
remove_action( 'woocommerce_before_shop_loop_item', 'woocommerce_template_loop_product_link_open', 10 );
// Re-add links and check for affiliate link
function myprefix_woocommerce_template_loop_product_link_open() {
$affiliate_link = get_post_meta( get_the_ID(), '_product_url', true );
if ( $affiliate_link ) {
echo '';
} else {
echo '';
}
}
add_action( 'woocommerce_before_shop_loop_item', 'myprefix_woocommerce_template_loop_product_link_open', 10 );