fix: sFileList will be 'undefined' when run form.resetFields()

[luichooy]

This is a ...

• New feature
• Bug fix
• Site / document update
• Component style update
• TypeScript definition update
• Refactoring
• Code style optimization
• Branch merge
• Other (about what?)

What's the background?

1. If combining Upload Component with Form Component,The Upload component will ocurr Error When running form.resetFields(). This is because that the code form.resetFields() will make fileList to undefined and Upload componenet watching fileList props to updatae sFileList.
2. Resolve what problem.
3. Issue: Upload组件与Form组件结合有问题 #925

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[codecov]

Codecov Report

Merging #929 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #929   +/-   ##
=======================================
  Coverage   87.27%   87.27%           
=======================================
  Files         145      145           
  Lines        4817     4817           
  Branches     1421     1422    +1     
=======================================
  Hits         4204     4204           
  Misses        570      570           
  Partials       43       43

Impacted Files	Coverage Δ
components/upload/Upload.jsx	78.02% <0%> (ø)	⬆️

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[github-actions]

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