fix: rendering issue due to missing default values

[ruibaby]

为验证码相关的配置添加默认值，以解决升级之后页面渲染异常的问题，需要注意，FormKit 的 group 类型不能只在其下输入框设置默认值，需要在 group 节点设置。

/kind bug

Fixes #137

修复缺失默认配置导致页面渲染异常的问题。

[f2c-ci-robot]

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[guqing]

建议撤销这些改动然后在

plugin-comment-widget/src/main/java/run/halo/comment/widget/SettingConfigGetter.java

Line 18 in 1549f84

private CaptchaConfig captcha;

给定初始化为 CaptchaConfig.empty() 即可，原因是序列化时没有值不会走 set 方法，这里忘记写默认值导致的

[JohnNiang]

/lgtm