✅ fix(test): 修复具体残疾部位字段空值测试期望

- 将测试期望从 undefined 改为 null，因为数据库字段 nullable: true
- 当字段为空时，数据库返回 null 而不是 undefined

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Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

allin-packages/disability-module/tests/integration/disability.integration.test.ts

@@ -243,7 +243,7 @@ describe('残疾人管理API集成测试', () => {
       if (response.status === 200) {
         const data = await response.json();
         expect(data.name).toBe(createData.name);
-        expect(data.specificDisability).toBeUndefined(); // 应该为undefined
+        expect(data.specificDisability).toBeNull(); // 应该为null，因为数据库字段nullable: true
       }
     });