Fix no usage_summary for services not active yet, minor improvements to traffic graph

app/Models/Service/Adsl.php

@@ -136,9 +136,9 @@ class Adsl extends ServiceType implements ServiceItem,ServiceUsage
 	/**
 	 * Find the last date any traffic was recorded for a service
 	 *
-	 * @return AdslTraffic
+	 * @return AdslTraffic|null
 	 */
-	private function usage_last_date(): AdslTraffic
+	private function usage_last_date(): ?AdslTraffic
 	{
 		return $this->traffic
 			->sortBy('date')