Adjust CastsAttributes::set's expected return type (#34142)

Author: xel1045

src/Illuminate/Contracts/Database/Eloquent/CastsAttributes.php

@@ -22,7 +22,7 @@ public function get($model, string $key, $value, array $attributes);
      * @param  string  $key
      * @param  mixed  $value
      * @param  array  $attributes
-     * @return array|string
+     * @return array|string|null
      */
     public function set($model, string $key, $value, array $attributes);
 }