Empty price_level in orderbook should be destoryed

[rm1138]

Should check the the price_level is empty or not after removing item from it.
eg: https://github.com/Origin-Byte/nft-protocol/blob/testnet/sources/trading/orderbook.move#L1259

Otherwise this will break the assumption when match_buy_with_ask_
eg: https://github.com/Origin-Byte/nft-protocol/blob/testnet/sources/trading/orderbook.move#L845

[porkbrain]

You're absolutely right, the price level should be deleted if empty.

[porkbrain]

    /// Finds an ask of a given NFT advertized for the given price. Removes it
    /// from the asks vector preserving order and returns it.
    fun remove_ask(asks: &mut CBTree<vector<Ask>>, price: u64, nft_id: ID): Ask {
        assert!(crit_bit::has_key(asks, price), EOrderDoesNotExist);

        let price_level = crit_bit::borrow_mut(asks, price);

        let index = 0;
        let asks_count = vector::length(price_level);
        while (asks_count > index) {
            let ask = vector::borrow(price_level, index);
            // on the same price level, we search for the specified NFT
            if (nft_id == ask.nft_id) {
                break
            };

            index = index + 1;
        };

        assert!(index < asks_count, EOrderDoesNotExist);

        let ask = vector::remove(price_level, index);

        if (vector::length(price_level) == 0) {
            // to simplify impl, always delete empty price level
            vector::destroy_empty(crit_bit::pop(asks, price_level));
        };

        ask
    }