Fix alerting bug handling null

Author: murrant

LibreNMS/Alerting/QueryBuilderParser.php

@@ -134,12 +134,11 @@ protected function findTablesRecursive($rules)
 
     /**
      * Initialize this from json generated by jQuery QueryBuilder
-     *
-     * @param  string|array  $json
-     * @return static
      */
-    public static function fromJson($json)
+    public static function fromJson(array|string|null $json): static
     {
+        $json ??= [];
+
         if (! is_array($json)) {
             $json = json_decode($json, true) ?: [];
         }