Backport Failed Check with Blank Diff

https://github.com/electron/electron/pull/37603/checks?check_run_id=12066937375

When manually backporting the cherry pick ran clean, so it's not clear why trop thinks it failed, and the blank diff makes it seem like it did not actually fail.

Start with the linked GitHub check run and compare its reported failure with the manually successful cherry-pick described in the issue. Trace the backport check handling in trop, then reproduce the blank-diff case. Done means the failure status and diff accurately reflect whether the backport actually failed.

Written by the indexing model from the issue text.