IcingaRedis: Do not fail while fetching last heartbeat

fixes #1372

Author: nilmerg

library/Icingadb/Common/IcingaRedis.php

@@ -188,7 +188,7 @@ public static function getLastIcingaHeartbeat(?Redis $redis = null)
         $rs = $redis->xread(1, null, [$stream], '0');
 
         if (is_array($rs)) {
-            $timestampKeyPos = array_search('timestamp', $rs[$stream][0][1], true);
+            $timestampKeyPos = array_search('timestamp', $rs[$stream][0][1] ?? [], true);
 
             if ($timestampKeyPos !== false && isset($rs[$stream][0][1][$timestampKeyPos + 1])) {
                 return $rs[$stream][0][1][$timestampKeyPos + 1] / 1000;