fix: onChange in cascader-view unable to get value

[1587315093]

resolved: #6442

[github-actions[bot]]

PR preview has been successfully built and deployed to https://antd-mobile-preview-pr-6444.surge.sh

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (671ea15) 92.10% compared to head (929140b) 92.28%. Report is 1 commits behind head on master.

Files	Patch %	Lines
...onents/cascader-view/use-cascader-value-extend.tsx	75.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #6444 +/- ## ========================================== + Coverage 92.10% 92.28% +0.18% ========================================== Files 316 316 Lines 6861 6866 +5 Branches 1719 1720 +1 ========================================== + Hits 6319 6336 +17 + Misses 504 494 -10 + Partials 38 36 -2 ```

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[zombieJ]

😎 缺测试~

[1587315093]

😎 缺测试~

done ~