Rasa: `rasa data split nlu` no longer works

[akelad]

Part of #6475

Rasa version: master

Running rasa data split nlu creates an empty train_test_split directory as of the 2.0 changes to the data format. It's supposed to generate test_data and training_data files (they used to be .json, but we should probably change that to be .yaml as well)

I tested this on the formbot and responseselectorbot examples, both with the same result.

Potentially related to this issue: https://github.com/RasaHQ/rasa/issues/6389

Whoever fixes it, please make sure to update the CLI documentation as well!

[m-vdb]

@akelad do you know why the default format was json in the first place?

[akelad]

it's archaic functionality from when json was the only format we had :D