[task scheduler] Fix /json/job endpoint for unknown job

Change-Id: Iad0be0ae9bdda4066ec645bb4f750a6a5d694d84
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/253178
Reviewed-by: Ben Wagner aka dogben <benjaminwagner@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
diff --git a/task_scheduler/go/task-scheduler-fe/main.go b/task_scheduler/go/task-scheduler-fe/main.go
index 0f477a2..9331182 100644
--- a/task_scheduler/go/task-scheduler-fe/main.go
+++ b/task_scheduler/go/task-scheduler-fe/main.go
@@ -310,6 +310,10 @@
 		httputils.ReportError(w, err, "Error retrieving Job.", http.StatusInternalServerError)
 		return
 	}
+	if job == nil {
+		http.Error(w, "Unknown Job", 404)
+		return
+	}
 
 	// Retrieve the task specs, so that we can include the task dimensions
 	// in the results.