has_next_page for indexpage should return true at least once

`has_next_page` should return at least once, even if continuation is not specified. This means that one can write more generic code with `while ip.has_next_page()`
No, this was the issue with `resp.continuation` being an empty string in the protobuf, even if the server had not sent it. An empty continuation string causes problems on the next request, whereas `None` signals the end of the query.