provide default demographics_transform if None given by researcher

MR description

Closes #429 (closed).

The suggested fix is to provide a default empty transform when the researcher did not provide one themselves. This allows researchers that only use demographics for filtering data to do so without additional (counterintuitive) effort.

On the other hand, this means that if someone had the expectation that they would be able to use their demographics data without specifying a transform, they are going to be disappointed. Hence I updated the comments in the notebook, and mirrored those changes in the docs (see fedbiomed.gitlabpages.inria.fr!70).

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