[SDK] Fix Base2ExponentialHistogramAggregation Merge with empty buckets

[dbarker]

Fixes #3424

Changes

• Add a test to cover merging two Base2ExponentialHistogramAggregation instances where one has empty buckets
• Update Base2ExponentialHistogramAggregation::Merge to shortcut if merging a histogram with no counts
• Check if buckets are empty before down scaling

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.06%. Comparing base (82ec2a5) to head (0832409).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3425      +/-   ##
==========================================
+ Coverage   90.05%   90.06%   +0.02%     
==========================================
  Files         212      212              
  Lines        6932     6939       +7     
==========================================
+ Hits         6242     6249       +7     
  Misses        690      690              

Files with missing lines	Coverage Δ
...egation/base2_exponential_histogram_aggregation.cc	90.91% <100.00%> (+0.28%)	⬆️

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[dbarker]

Hi @ethandmd, Could you help review this PR? Thanks!

[ethandmd]

lgtm, thanks @dbarker!

[lalitb]

Thanks.