Fix bug preventing overwriting attrs with a blank list

[JonnyWong16]

Description

Explicitly specify the Playable session attributes that should not be overwritten with a blank list.

Fixes #660

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable