FIX: treat None in datetime_converter

src/caosadvancedtools/table_importer.py

@@ -88,7 +88,10 @@ def date_converter(val, fmt="%Y-%m-%d"):
     converts it using format string
     """
 
-    return datetime_converter(val, fmt=fmt).date()
+    if val is None:
+        return None
+    else:
+        return datetime_converter(val, fmt=fmt).date()
 
 
 def incomplete_date_converter(val, fmts={"%Y-%m-%d": "%Y-%m-%d",