Meaningful error message if sample splitting was not yet set

[MalteKurz]

Calling

dml_plr_obj = DoubleMLPLR$new(make_plr_CCDDHNR2018(),
                                                        lrn("regr.ranger"), lrn("regr.ranger"),
                                                        draw_sample_splitting = FALSE)
dml_plr_obj$fit()

produces error message

 Error in .__ResamplingCustom__instantiate(self = self, private = private,  : 
  Assertion on 'train_sets' failed: Must be of type 'list', not 'NULL'. 

More meaningful would be something in the lines of https://github.com/DoubleML/doubleml-for-py/blob/a574e0afcab0e7cce475925f1344399e75dd4a11/doubleml/double_ml.py#L238-L239.