Comment 9 for bug 1748001

By far the simplest way to resolve this seems to be just removing these lines from the `setEngineKey` code:

```
    if (thisFileKey == mixxx::track::io::key::INVALID ||
        newKey == mixxx::track::io::key::INVALID) {
        return;
    }
```

Removing it fixes the issue and doesn't appear to break anything else. I can't see any ill effects of running setEngineKey when the file or destination key are zero.

Have commented in more depth on Zulip: https://mixxx.zulipchat.com/#narrow/stream/109171-development/subject/Reset.20key.20button.2Ftracks.20with.20no.20key/near/130053817