[SDK-239] Fix dataset types

[mnoszczak]

Makes the types more correct

[kopreschko]

lgtm

[vbrodsky]

Not sure we need Optional here... please see this too https://stackoverflow.com/questions/69440494/python-3-10-optionaltype-or-type-none#:~:text=Optional%20was%20invented%20to%20indicate,gets%20rid%20of%20an%20identifier.

[vbrodsky]

Also here we have a default None, so Optional is not required:
https://docs.python.org/3/library/typing.html#typing.Optional

[mnoszczak]

@vbrodsky:

PEP 484 prohibits implicit Optional. Accordingly, mypy has changed its default to no_implicit_optional=True. Use https://github.com/hauntsaninja/no_implicit_optional to automatically upgrade your codebasemypynote

[vbrodsky]

this is tricky, but correct change I believe, based on https://docs.python.org/3/library/typing.html#typing.Optional