chore: Remove no-op function in plugin config logic

[TrevorBurnham]

Description

Per previous discussion at #8487, this PR removes the removeUndefinedValues utility function added by #2281, which had a bug that made it effectively a no-op. The issue reported at #2153 somehow hasn't resurfaced in 3 years.

I've also removed the ?? {} after viteConfig.test: deepMerge treats undefined values and empty objects as equivalent, so there's no reason to allocate an extra object here.

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[sheremet-va]

Seems like it does do something (CI is failing)

[TrevorBurnham]

Seems like it does do something (CI is failing)

Weird… I thought the CI failures were random, but I rebased and see the same failures. And they're consistent across all OSes the CI runs against:

@vitest/test-config : test test/config
  test/config test$ vitest --typecheck.enabled
   ELIFECYCLE  Command failed with exit code 1.

In local dev, if I run the test/config tests (cd test/config && pnpm test), all tests pass. I get failures on my machine when I run pnpm run test:ci, but they're unrelated failures (random timeouts in istanbul and `istanbul-browser).

Any idea why that error might be happening in CI? It's weird that there isn't even any output before the process exits, right? 😕