fix(theme-generator): shadow setting is null

[RSS1102]

🤔 这个 PR 的性质是？

• 日常 bug 修复
• 新特性提交
• 文档改进
• 演示代码改进
• 组件样式/交互改进
• CI/CD 改进
• 重构
• 代码风格优化
• 测试用例
• 分支合并
• 其他

🔗 相关 Issue

💡 需求背景和解决方案

📝 更新日志

• fix(theme-generator): 阴影设置时变量为 null

• 本条 PR 不需要纳入 Changelog

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[tencent-adm]

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npm i https://pkg.pr.new/Tencent/tdesign/@tdesign/theme-generator@792

commit: f92efdc

[tdesign-bot]

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[uyarn]

这是修的啥..

[RSS1102]