Use a defaultdict for log rendering

src/pretix/control/logdisplay.py

@@ -1,4 +1,5 @@
 import json
+from collections import defaultdict
 from decimal import Decimal
 
 import bleach
@@ -281,6 +282,7 @@ def pretixcontrol_logentry_display(sender: Event, logentry: LogEntry, **kwargs):
             data['value'] = LazyI18nString(data['value'])
 
     if logentry.action_type in plains:
+        data = defaultdict(lambda: '?', data)
         return plains[logentry.action_type].format_map(data)
 
     if logentry.action_type.startswith('pretix.event.order.changed'):