fix(perf): use N/A for missing benchmark fields to fix BigQuery schema validation

[Mahalaxmibejugam]

This PR fixes a bug where missing fields in benchmarks (such as batch_size added by the cat_ranges microbenchmarks) were serialized as empty strings instead of "N/A". This caused BigQuery schema validation to fail for INT64 columns. BigQuery expects missing numerical fields to be marked as N/A per the table configuration nullMarker: "N/A".

[gemini-code-assist]

Code Review

This pull request updates benchmark reporting in both microbenchmarks and subsystem benchmarks to use "N/A" as a default placeholder value instead of empty strings when benchmark metadata, extra info, stats, or percentile values are missing or empty. The feedback suggests simplifying the checks for None and empty strings to the more idiomatic Python expression val not in (None, "") across both modified files to improve readability.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.25%. Comparing base (060d20a) to head (c28672a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1065   +/-   ##
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  Coverage   90.25%   90.25%           
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  Files          16       16           
  Lines        3755     3755           
=======================================
  Hits         3389     3389           
  Misses        366      366           

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[zhixiangli]

Does "nullMarkers": ["N/A", ""] help?

[Mahalaxmibejugam]

Does "nullMarkers": ["N/A", ""] help?

We could use a "" NullMarker, but if any fields in future contain a legitimate "", they would be parsed as NULL, making it impossible to distinguish between the two. So handling it at python layer might be better? WDYT?