fix: type hinting in collection.py

[Sugihiru]

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

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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!