Fix alert history graph when empty (librenms#19751)

* Fix alert history graph when empty

* Update print-graph-alerts.inc.php

---------

Co-authored-by: Neil Lathwood <gh+n@laf.io>

Author: murrant

includes/html/print-graph-alerts.inc.php

@@ -102,10 +102,10 @@
         },
         zoomMin: 86400, //24hrs
         zoomMax: <?php
-        $first_date = reset($data);
-        $last_date = end($data);
-        $milisec_diff = abs(strtotime((string) $first_date['x']) - strtotime((string) $last_date['x'])) * 1000;
-        echo $milisec_diff;
+        $firstDate = reset($data)['x'] ?? null;
+        $lastDate = end($data)['x'] ?? null;
+        $millisecondDiff = ($firstDate && $lastDate) ? abs(strtotime((string) $firstDate) - strtotime((string) $lastDate)) * 1000 : 0;
+        echo $millisecondDiff;
         ?>,
         orientation:'top'
     };