Sensu: Correct type hint (#13915)

* Correct type annotation

Not 100% sure this is correct as I don't know what the previous change was trying to correct - but they're definitely not all strings.

* Correct the correction

LibreNMS/Alert/Transport/Sensu.php

@@ -151,7 +151,7 @@ class Sensu extends Transport
             'librenms-device-id' => strval($obj['device_id']),
             'librenms-rule-id' => strval($obj['rule_id']),
             'librenms-status-reason' => $obj['status_reason'],
-        ], function (string $s): bool {
+        ], function (?string $s): bool {
             return (bool) strlen($s); // strlen returns 0 for null, false or '', but 1 for integer 0 - unlike empty()
         });
     }