learner imputation can lead to unexpected behaviour

[sebffischer]

https://stackoverflow.com/q/73789223/8167262

This is caused by https://github.com/mlr-org/mlr3pipelines/blob/ed878ca735dc23e1f48cf4f6339a01b82b74c94d/R/PipeOpImputeLearner.R#L164

I don't fully understand the comment however.

Maybe this should be documented better.