minor fix for transformation, if no unfiltered branch is used

mlair/data_handler/data_handler_mixed_sampling.py

@@ -423,12 +423,18 @@ class DataHandlerMixedSamplingWithClimateAndFirFilter(DataHandlerMixedSamplingWi
 
         # combine all transformations
         transformation_res = {}
-        if len(transformation_chem) > 0:
-            transformation_res["filtered_chem"] = transformation_chem.pop("filtered")
-            transformation_res["unfiltered_chem"] = transformation_chem.pop("unfiltered")
-        if len(transformation_meteo) > 0:
-            transformation_res["filtered_meteo"] = transformation_meteo.pop("filtered")
-            transformation_res["unfiltered_meteo"] = transformation_meteo.pop("unfiltered")
+        if isinstance(transformation_chem, dict):
+            if len(transformation_chem) > 0:
+                transformation_res["filtered_chem"] = transformation_chem.pop("filtered")
+                transformation_res["unfiltered_chem"] = transformation_chem.pop("unfiltered")
+        else:  # if no unfiltered chem branch
+            transformation_res["filtered_chem"] = transformation_chem
+        if isinstance(transformation_meteo, dict):
+            if len(transformation_meteo) > 0:
+                transformation_res["filtered_meteo"] = transformation_meteo.pop("filtered")
+                transformation_res["unfiltered_meteo"] = transformation_meteo.pop("unfiltered")
+        else:  # if no unfiltered meteo branch
+            transformation_res["filtered_meteo"] = transformation_meteo
         return transformation_res if len(transformation_res) > 0 else None
 
     def get_X_original(self):