Graphite missing hostname fix (librenms#17955)

Author: murrant

LibreNMS/Data/Store/Graphite.php

@@ -91,7 +91,7 @@ public function write(string $measurement, array $fields, array $tags = [], arra
 
         // metrics will be built as prefix.hostname.measurement.field value timestamp
         // metric fields can not contain . as this is used by graphite as a field separator
-        $hostname = preg_replace('/\./', '_', $hostname);
+        $hostname = preg_replace('/\./', '_', (string) $hostname);
         $measurement = preg_replace(['/\./', '/\//'], '_', $measurement);
         $measurement = preg_replace('/\|/', '.', $measurement);