docs: fix @var

$DBGroup is nullable.

Author: kenjis

system/BaseModel.php

@@ -63,8 +63,8 @@ abstract class BaseModel
      * The Database connection group that
      * should be instantiated.
      *
-     * @var string
-     * @phpstan-var non-empty-string
+     * @var string|null
+     * @phpstan-var non-empty-string|null
      */
     protected $DBGroup;