fix(Calendar): fix watch 'defaultDate' error

[pany-ang]

当 defaultDate 为 undefined 时，现有的 watch 会将其重置为 null，逻辑如下：

    watch(
      () => props.defaultDate,
      (value = null) => {
        currentDate.value = value;
        scrollToCurrentDate();
      },
    );

而在该日历组件中，undefined 表示「今天」，null 表示 「不选择」。如果业务层将 defaultDate 重置为 undefined，这时候日历组件将会表现为「不选择」，业务层必须手动调用一次 reset 方法才能实现日历上选中「今天」

所以该侦听器逻辑应该是：

    watch(
      () => props.defaultDate,
      (value) => {
        if (value === undefined) {
          value = getInitialDate();
        }
        currentDate.value = value;
        scrollToCurrentDate();
      },
    );

简化一下：

    watch(
      () => props.defaultDate,
      (value) => {
        reset(value);
      },
    );

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.67%. Comparing base (eef8e0b) to head (0b27cf3).
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13077      +/-   ##
==========================================
+ Coverage   89.66%   89.67%   +0.01%     
==========================================
  Files         257      257              
  Lines        6987     6985       -2     
  Branches     1724     1724              
==========================================
- Hits         6265     6264       -1     
+ Misses        382      381       -1     
  Partials      340      340              

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[chenjiahan]

方便加个单元测试嘛?

[pany-ang]

单测已添加

[chenjiahan]

👍🏻