Empty log snippets should not be guessed

[DanielEScherzer]

See rust-lang/rust#159300 (comment)

The job **`tidy`** failed! Check out the build log: [(web)](https://github.com/rust-lang/rust/actions/runs/29359417502/job/87175522268) [(plain enhanced)](https://triage.rust-lang.org/gha-logs/rust-lang/rust/87175522268) [(plain)](https://github.com/rust-lang/rust/commit/6eeacfca28f1a5ee177cbb1a6e95bb328bd11fe5/checks/87175522268/logs)

<details><summary><i>Click to see the possible cause of the failure (guessed by this bot)</i></summary>

```plain

```

</details>

If the bot can't make a guess, it should say so instead of having an empty details that wastes developer time on expanding it