perf: cache empty optimizer result

[patak-dev]

Description

When an app doesn't have dependencies, we have been dropping the empty optimization result (after include+scanner optimization). This means that for these apps, the scanner is always run. Even if an app without deps is rare, I think we should still cache the empty results. This also affects warm start benchmarks for apps like vite-dev-server-perf

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

[stackblitz]

Run & review this pull request in StackBlitz Codeflow.

[patak-dev]

/ecosystem-ci run

[vite-ecosystem-ci]

📝 Ran ecosystem CI on 84a1574: Open

suite	result	latest scheduled
analogjs	✅ success	✅ success
astro	✅ success	✅ success
histoire	✅ success	✅ success
ladle	✅ success	✅ success
laravel	❌ failure	❌ failure
marko	✅ success	✅ success
nuxt	❌ failure	❌ failure
nx	✅ success	✅ success
previewjs	✅ success	✅ success
qwik	✅ success	✅ success
rakkas	✅ success	✅ success
sveltekit	✅ success	✅ success
unocss	✅ success	✅ success
vike	✅ success	✅ success
vite-plugin-pwa	✅ success	✅ success
vite-plugin-react	✅ success	✅ success
vite-plugin-react-pages	✅ success	✅ success
vite-plugin-react-swc	✅ success	✅ success
vite-plugin-svelte	✅ success	✅ success
vite-plugin-vue	✅ success	✅ success
vite-setup-catalogue	✅ success	✅ success
vitepress	✅ success	✅ success
vitest	✅ success	✅ success