#4867 fix/mutable default argument

A description of the changes proposed in the pull request:

https://docs.python-guide.org/writing/gotchas/#mutable-default-arguments

Basically, using mutable default types can lead to some really odd behaviors and should be avoided at all costs!

Tests performed by the developer:

Tips for QA testing:

See the guide I linked.

PR Link: https://github.com/hummingbot/hummingbot/pull/4867