fix: help will shake in Form

[zombieJ]

🤔 This is a ...

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

🔗 Related issue link

fix #20813

💡 Background and solution

📝 Changelog

Language	Changelog
🇺🇸 English	Fix Form.Item change help makes layout shake.
🇨🇳 Chinese	修复 Form.Item 改变 help 会导致布局闪动的问题。

☑️ Self Check before Merge

• Doc is updated/provided or not needed
• Demo is updated/provided or not needed
• TypeScript definition is updated/provided or not needed
• Changelog is provided or not needed

[tests-checker]

Could you please add tests to make sure this change works as expected?

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[codecov]

Codecov Report

Merging #20886 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #20886      +/-   ##
==========================================
+ Coverage   97.47%   97.47%   +<.01%     
==========================================
  Files         296      296              
  Lines        6883     6887       +4     
  Branches     1911     1913       +2     
==========================================
+ Hits         6709     6713       +4     
  Misses        174      174

Impacted Files	Coverage Δ
components/form/FormItemInput.tsx	93.1% <ø> (ø)	⬆️
components/form/util.ts	100% <100%> (ø)	⬆️

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