FIX: treat None in datetime_converter

src/caosadvancedtools/table_importer.py

@@ -88,6 +88,9 @@ def date_converter(val, fmt="%Y-%m-%d"):
     converts it using format string
     """
 
+    if val is None:
+        return None
+    else:
         return datetime_converter(val, fmt=fmt).date()