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test-exempt: refactor with no semantic change

When running benchmarks on triple-headed builds (tip-of-tree of framework, engine, and Dart SDK), the gitBranch parameter is null.
Upload to cocoon is a separate step in the drone recipe, that only runs if an builder property is set, and so only runs when this is non-null. But there are runs of the drone recipe that only upload the results file to CAS, with a null gitBranch (CommitBranch in the json).

So: the assert non-null in cocoon.dart:118 is incorrect, but was never run in production
The gitBranch parameter must be nullable in writeTaskResultToFile

The check of gitBranch for non-null added in line 45 should not be added.

An example of a build uploading json with a null value for CommitBranch is
https://luci-milo.appspot.com/ui/p/flutter/builders/try/Linux_staging%20hot_mode_dev_cycle_linux__benchmark/232
and the uploaded json can be seen through the CAS UI on the upload to cas step.