fix: modulepreload polyfill only during build (fix #4786)

[sapphi-red]

Description

I changed the content of vite/modulepreload-polyfill to '' not during dev.
This will fix #4786.

I think this change can be justified for the following reason.

• polyfill is not necessary during dev
  • also even if this polyfill is not included and modulepreload is not supported, the code works unlike other polyfills
• the current polyfill includes __VITE_IS_MODERN__ and it only works during build
  • it can be solved by avoiding this by replacing this even not during build, but I don't think it is needed

fixes #4786
close #6507

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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