OSB-34: fix in case of None

LCU/Maintenance/DBInterface/monitoringdb/views/rtsm_views.py

@@ -49,7 +49,7 @@ class RTSMSummaryPlot(viewsets.ViewSet):
             check_error_summary_plot.delay(pk)
             raise Http404()
 
-        if os.path.exists(uri) and os.path.isfile(uri):
+        if uri and os.path.exists(uri) and os.path.isfile(uri):
                 with open(uri, 'rb') as f_stream:
                     image = f_stream.read()
                 return HttpResponse(image, status=status.HTTP_200_OK, content_type='image/gif')