Для слабых
v-if="this.$store.state.cart.cart.cart_items && this.$store.state.cart.cart.cart_items.length == 1"
Для сильных: создай computed
computed: {
getItemsLength(){
return this.$store.state.cart.cart.cart_items?.length == 1
}
}
В шаблоне
v-if="getItemsLength"