fix(sourcemap): combine sourcemaps with multiple sources without matched source

[sapphi-red]

Description

fixes #18957
refs #17677

[pkg-pr-new]

Open in Stackblitz

npm i https://pkg.pr.new/vite@18971

commit: d64630e

[sapphi-red]

/ecosystem-ci run

[vite-ecosystem-ci]

📝 Ran ecosystem CI on d64630e: Open

suite	result	latest scheduled
histoire	❌ failure	❌ failure
laravel	❌ failure	✅ success
marko	❌ failure	✅ success
redwoodjs	❌ failure	❌ failure
remix	❌ failure	❌ failure
vike	❌ failure	❌ failure
vite-plugin-svelte	❌ failure	❌ failure
waku	❌ failure	❌ failure

✅ analogjs, astro, ladle, nuxt, previewjs, quasar, qwik, rakkas, storybook, sveltekit, unocss, vite-environment-examples, vite-plugin-pwa, vite-plugin-react, vite-plugin-react-swc, vite-plugin-vue, vite-setup-catalogue, vitepress, vitest, vuepress

[Santana1053]

This appears to fix the sourcemap issue in my environment.