Fix paginator bug where `None` was returned immediately (#1083)

* Fix paginator bug where `None` was returned immediately

The escape hatch added by aws-sdk-rust#391 did not properly handle the case where the first response was `None` and _not_ the empty string. This diff:
- Checks for emptiness for both maps and strings
- Fixes the check so that an initial `None` input does not cause an incorrect paginator error

* Update changelog

* Apply suggestions from code review

Co-authored-by: Zelda Hessler <zhessler@amazon.com>

* rustfmt

Co-authored-by: Zelda Hessler <zhessler@amazon.com>