refactor: enable nullable

[Jaxelr]

Description

This PR enables nullable on the entire codebase.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

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 made corresponding changes to the documentation (these include code samples)
• My changes generate no new warnings (if so, why?)

[Jaxelr]

I still need to make tests and samples nullable enable.

[Jaxelr]

I still need to make tests and samples nullable enable.

This is all done.