Cargo will always output something on success:
```
$ cargo check --message-format=json
{"reason":"compiler-artifact", ... snipped ... }
{"reason":"build-finished","success":true}
```
However, rustc does not output anything on success:
```
$ rustc --error-format=json main.rs
$ echo $?
0
```
Restore the behaviour prior to #10517, where an exit code of 0 is
considered good even if nothing is written to stdout.
This enables custom overrideCommand values that use rustc rather than
cargo.
f6f6b3a7ad