warn if there's an empty flow description (#831)

* warn if there's an empty flow description

* Use logging string interpolation.

Author: amueller

openml/flows/flow.py

@@ -1,6 +1,7 @@
 from collections import OrderedDict
 import os
 from typing import Dict, List, Union  # noqa: F401
+import logging
 
 import xmltodict
 
@@ -223,6 +224,10 @@ def _to_dict(self) -> dict:
             _add_if_nonempty(flow_dict, 'oml:{}'.format(attribute),
                              getattr(self, attribute))
 
+        if not self.description:
+            logger = logging.getLogger(__name__)
+            logger.warn("Flow % has empty description", self.name)
+
         flow_parameters = []
         for key in self.parameters:
             param_dict = OrderedDict()  # type: 'OrderedDict[str, str]'