fix(docstore/awsdynamodb): ensure Next returns EOF when no more items

[bartventer]

Fixes #3405

Although I was unable to reproduce the panic, I added a preventive check for awsdynamodb here. This check returns io.EOF if there are no more items, which should prevent an index out of range panic when Next is called after it has already returned EOF once.

I also added a test case to verify this behavior. The test repeatedly calls Next until it gets io.EOF, then calls Next again to ensure it still returns io.EOF.

I also generated new replay files for awsdynamodb and gcpfirestore.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 73.12%. Comparing base (1f3e996) to head (daa25e8).

Files	Patch %	Lines
docstore/awsdynamodb/query.go	0.00%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3406 +/- ## ========================================== - Coverage 73.13% 73.12% -0.01% ========================================== Files 113 113 Lines 14862 14864 +2 ========================================== Hits 10870 10870 - Misses 3218 3219 +1 - Partials 774 775 +1 ```

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