fix: fix external: true merging

[hi-ogawa]

I noticed a following config crashes on rolldown-vite
https://stackblitz.com/edit/vitejs-rolldown-vite-bq4yd9u3?file=vite.config.ts

import { defineConfig } from 'vite';

export default defineConfig({
  ssr: {
    external: true,
  },
  environments: {
    ssr: {
      resolve: {
        external: ['node:path'],
      },
    },
  },
});

$ vite
...
error when starting dev server:
Error: Value is non of these types `True`, `Array<T>`,  on BindingViteResolvePluginConfig.external
    at napi_create_error (/tmp/rolldown-1.0.0-beta.52/node_modules/.pnpm/@emnapi+core@1.7.1/node_modules/@emnapi/core/dist/emnapi-

I think this is rather unusual setup, but Vitest happens to have a test test failing due to this vitest-dev/vitest#9121 (comment).

[sapphi-red]

/ecosystem-ci run

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/vite@21164

commit: 70d41bb

[vite-ecosystem-ci]

📝 Ran ecosystem CI on bccd855: Open

suite	result	latest scheduled
analogjs	❌ failure	❌ failure
astro	❌ failure	❌ failure
react-router	✅ success	⏹️ cancelled
qwik	❌ failure	❌ failure
unocss	✅ success	❌ failure
storybook	❌ failure	❌ failure
vike	❌ failure	❌ failure
vite-plugin-cloudflare	❌ failure	❌ failure

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