docstore/awsdynamodb: Fix Query for cases where the underlying query to AWS returns an empty set, but there's still more data

[vangent]

Fixes #3405

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 66.66667% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 73.16%. Comparing base (3f641fd) to head (cb56d76).

Files	Patch %	Lines
docstore/awsdynamodb/query.go	66.66%	2 Missing and 2 partials :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3408      +/-   ##
==========================================
+ Coverage   73.15%   73.16%   +0.01%     
==========================================
  Files         113      113              
  Lines       14872    14874       +2     
==========================================
+ Hits        10880    10883       +3     
  Misses       3218     3218              
+ Partials      774      773       -1     

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