stream settings: Fix brand new streams showing 0 weekly traffic.

A stream created in the last few hours likely won't be in StreamCount
(since that gets updated once a day), and hence won't be in the
recent_traffic dict.

However, get_average_weekly_stream_traffic should be None in this case,
not 0.

zerver/lib/actions.py

@@ -3969,7 +3969,7 @@ def get_average_weekly_stream_traffic(stream_id: int, stream_date_created: datet
     try:
         stream_traffic = recent_traffic[stream_id]
     except KeyError:
-        return 0
+        stream_traffic = 0
 
     stream_age = (timezone_now() - stream_date_created).days