fix: Null to string

(cherry picked from commit b2164e6)

src/orm/illuminate/Log.php

@@ -109,7 +109,7 @@ public function getOldValue($model, string $key): string
 
         $val = json_decode($value, true);
         if (!isset($jsonKey) || is_null($val) || !is_array($val)) {
-            return $value;
+            return (string) $value;
         }
 
         foreach (explode('->', $jsonKey) as $k) {