Fix null tracker metadata and disable mlflow by default

[gmertes]

• This PR fixes the tracker metadata to be empty when there is no tracker, instead of "null".
• Since the tracking URI is now missing by default, I also changed mlflow to be disabled by default, so that the default training config does not crash.

When tracking is disabled, the metadata contains the following for the tracker:

"tracker": {
    "null": null
}

Instead, it should be an empty object to indicate there was no tracker:

"tracker": {}

[FussyDuck]

All committers have signed the CLA.

[JesperDramsch]

Very nice.

I think we should probably have a check that W&B and MLFlow aren't both enabled, as this would break the tracker info though.

[gmertes]

I agree. Since I don't think both trackers should be enabled at the same time, that check could be part of the config validation proposed in #1