utility.presence: fix for activities that send empty timestamp objects

*cough* tower unite *cough*

src/modules/utility.js

@@ -838,7 +838,10 @@ presence.callback = async function (msg, line) {
             descLines.push(
               formatTime(activity.timestamps.end - Date.now()) + " remaining"
             );
-          } else {
+          } else if (
+            activity.timestamps.start != null &&
+            activity.timestamps.end != null
+          ) {
             const position = Date.now() - activity.timestamps.start;
             const length = activity.timestamps.end - activity.timestamps.start;