chore: optimize experience of hover value

[kerm1it]

close ant-design/ant-design#29412

在 basic 那个例子的 Uncontrolled DateTime 查看效果

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[codecov]

Codecov Report

Merging #220 (29216be) into master (6af42f8) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #220   +/-   ##
=======================================
  Coverage   99.52%   99.52%           
=======================================
  Files          45       45           
  Lines        2104     2113    +9     
  Branches      619      623    +4     
=======================================
+ Hits         2094     2103    +9     
  Misses          8        8           
  Partials        2        2           

Impacted Files	Coverage Δ
src/panels/DatetimePanel/index.tsx	100.00% <ø> (ø)
src/PickerPanel.tsx	100.00% <100.00%> (ø)
src/utils/timeUtil.ts	100.00% <100.00%> (ø)

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[kerm1it]

先别合，有点问题

[kerm1it]

优化了 viewDate 中 postDate 的逻辑。

之前：

当前的 viewDate 为空时返回当前时间，因此当未选择时，切换 date 会看到 placeholder 是动态的，一旦 viewDate 固定了，就会返回固定的 viewDate（切换月份、年份面板导致）

之后:

viewDate 为空时逻辑保持不变，当不为空时加了一层判断：如果当前 value 为空且设置了 showTime 选项时，viewDate 中的 time 按照 showTime.defaultValue > defaultValue > now 优先级顺序设置为对应的 time。

@afc163 @zombieJ 看看有没有问题？