Hide task duration if we don't have a start_date

In some failure scenarios, we can have an end_date without a start_date.
If we naively show a duration in these cases, we end up with a negative
duration, which isn't ideal. This hides the duration if we don't have
start_date.

End_date wasn't added to the condition so a duration is still shown for
running tasks - those with a start_date but no end_date.

Author: jedcunningham

airflow/ui/src/pages/TaskInstance/Header.tsx

@@ -46,7 +46,9 @@ export const Header = ({
     ...(taskInstance.try_number > 1 ? [{ label: "Try Number", value: taskInstance.try_number }] : []),
     { label: "Start", value: <Time datetime={taskInstance.start_date} /> },
     { label: "End", value: <Time datetime={taskInstance.end_date} /> },
-    { label: "Duration", value: `${getDuration(taskInstance.start_date, taskInstance.end_date)}s` },
+    ...(Boolean(taskInstance.start_date)
+      ? [{ label: "Duration", value: `${getDuration(taskInstance.start_date, taskInstance.end_date)}s` }]
+      : []),
     {
       label: "DAG Version",
       value: