fix: type hinting in collection.py

[Sugihiru]

Mostly adding | None on all fields that may have None as a default value.

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes#687 🦕

[Mariatta]

Thank you.

[Mariatta]

The | syntax is only available in Python 3.10+. Since we still support Python 3.7+, then this should be using Union instead of |.

e.g

from typing import Union
...
document_id: Union[str, None]

[Mariatta]

Thanks!