schedulelib: Ignore missing track field

According to the XML schema, the "track" field of an event is not mandatory. So we should replace it with an empty string if not existing like we do for other fields.

schedulelib.py

@@ -100,6 +100,12 @@ def events(scheduleUrl, titlemap={}):
                     url = event.find('url').text.strip()
                 else:
                     url = ''
+
+                if event.find('track') is not None and event.find('track').text is not None:
+                    track = event.find('track').text
+                else:
+                    track = ''
+
                 # yield a tupel with the event-id, event-title and person-names
                 yield {
                     'day': day.get('index'),
@@ -109,7 +115,7 @@ def events(scheduleUrl, titlemap={}):
                     'persons': personnames,
                     'personnames': ', '.join(personnames),
                     'room': room.attrib['name'],
-                    'track': event.find('track').text,
+                    'track': track,
                     'url': url
                 }