#5108 Fix TWAP strategy default values don't make sense

A fix of following issue:

• order price optional in strategy parameters - order price needs to be always provided, can't be optional
• default values of is_buy, target_asset_amount, order_step_size, order_price are not justified, "1" is not an universally correct value, and the parameters always have to be provided by the user

Link: https://github.com/hummingbot/hummingbot/pull/5108