Check max_rung is None not if it returns false

If max_rung==0 is not None

src/python/py_interface/import_dataframe.py

@@ -140,7 +140,7 @@ def read_csv(
             max_rung = inputs.max_rung
     else:
         inputs_out = Inputs()
-    if not max_rung:
+    if max_rung is None:
         raise ValueError("Maximum rung for the calculation is not defined.")
 
     # Load csv file