Make unavailable attributes default to None

[Nubuki-all]

For example trying to access anime.title.english shouldn't result in an AttributeError when anilist doesn't return one
It should instead return None
The current way to check if the value exists is with hasattr which is mostly inconvenient

[AndrielFR]

You are right, I felt this need and problem when rewriting to Rust, I will provide the changes soon