fix error when showning status view right after server start

app/Http/Controllers/MarkController.php

@@ -181,6 +181,12 @@ class MarkController extends Controller
      */
     private function computeExecutionRate(array $history_jobs_executed) : array
     {
+
+        if (count($history_jobs_executed) < 2) {
+            // not enough points to compute execution rate...
+            return [];
+        }
+
         $points = [];
         $last_point_time = $history_jobs_executed[0]->t;
         $last_point_value = $history_jobs_executed[0]->y;