fix: sourcemap source point to null

[poyoho]

Description

fix: #8289 (comment)

For example:
if the file just transform by importMetaGlob plugin it will got an error.

Additional context

in dev mode if plugin no generate map field, will generate by middleware.

https://github.com/vitejs/vite/blob/main/packages/vite/src/node/server/pluginContainer.ts#L649

I think we can resolve it by setting generateMap({ source: id }) like pluginContainer default generate sourcemap.

force set build.sourcemap === true when in dev mode, but I think force setting of this value may confuse users when resolvingConfig hook

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[sapphi-red]

LGTM 👍