Make ruff format idempotent when using stdin input (#7581)
## Summary
Currently, this happens
```sh
$ echo "print()" | ruff format -
#Notice that nothing went to stdout
```
Which does not match `ruff check --fix - ` behavior and deletes my code
every time I format it (more or less 5 times per minute :smile:).
I just checked that my example works as the change was very
straightforward.
