Fix sklearn tree importances retrieval bug (#376)

When obtaining the tree feature kpis we access the JSON report path
`treePreparationReport.variablesStatistics`. If there are informative
variables but no informative tree `variablesStatistics` is not defined.
We didn't check that and it lead to a bug in this rare case.

Author: folmos-at-orange

khiops/sklearn/estimators.py

@@ -1435,7 +1435,11 @@ def _fit_training_post_process(self, ds):
         else:
             pair_feature_evaluated_names_ = []
             pair_feature_evaluated_levels_ = []
-        if "treePreparationReport" in self.model_report_.json_data:
+        if (
+            "treePreparationReport" in self.model_report_.json_data
+            and "variablesStatistics"
+            in self.model_report_.json_data["treePreparationReport"]
+        ):
             tree_preparation_report = self.model_report_.json_data[
                 "treePreparationReport"
             ]["variablesStatistics"]