context_processors: prevent a null station error when no default station is defined

aircox/context_processors/__init__.py

@@ -1,3 +1,4 @@
 def station(request):
     station = request.station
-    return {"station": station, "audio_streams": station.streams}
+    audio_streams = station.streams if station else None
+    return {"station": station, "audio_streams": audio_streams}