Fix pagination of empty response for PaginationType.NONE

[dtatarkin]

Library raise StopIteration if endpoint returns an empty list while handle_pagination is PaginationType.NONE

[hiohiohio]

@dtatarkin thank you for this PR.
I have noticed that PaginationType.NONE might not work and should not be used without fixed for the current methods/responses.
The issue is that alpaca-py does not return page_token in a response of methods. Therefore, it might easily cause an issue of not noticing actual length of records/data. It might cause a breaking change as well. In the case, raw data/response could be dict instead of list.
I am now considering removing/deprecating PagenationType.NONE rather than fixing with the current spec as we might not have a way to return page_token in a raw data format.

fyi. the other consideration is that this change causes the behaviour change and might cause surprise to existing users. But I wanted to point out the issue above more.