Build fails with custom theme and no css

[domnantas]

Describe the bug
I started building a custom theme and haven't imported any css to index.js yet. Running vitepress build throws an error:

$ vitepress build
vitepress v0.11.4
vite v2.0.0-beta.35
✓ building client + server bundles...
✖ rendering pages...
build error:
 TypeError: Cannot read property 'fileName' of undefined
    at Object.renderPage (/Users/fistmenaruto/Projects/domnantas.lt/node_modules/vitepress/dist/node/build/render.js:53:61)
    at async Object.build (/Users/fistmenaruto/Projects/domnantas.lt/node_modules/vitepress/dist/node/build/build.js:29:17)
error Command failed with exit code 1.

render.js:53 is trying to get cssChunk.fileName:

<link rel="stylesheet" href="${siteData.base}${cssChunk.fileName}">

but it is undefined since there are not css chunks.

The same happens if you try to import an empty .css file to index.js.

vitepress dev does not throw the error.

To Reproduce
Steps to reproduce the behavior:

• Create an empty custom theme as described in the docs: https://vitepress.vuejs.org/guide/customization.html#customization
  (notice how there are no .css imports in the docs either)
• Run vitepress build

Reproduction repo: https://github.com/domnantas/domnantas.lt/tree/no-css-chunks

Expected behavior
Build should not fail when building without css

System Info

• vitepress version: v0.11.4
• vite version: v2.0.0-beta.35
• Node version: v15.5.1
• OS version: MacOS 11.0.1

Additional context

`
...
${cssChunk ? `<link rel="stylesheet" href="${siteData.base}${cssChunk.fileName}">` : ""}
...
`

would fix this, but maybe there are better ways

[kiaking]

Good catch! Thanks for the report. I think your fix is reasonable. If there're no css files, we should just stop requiring it. Would you like to make a PR? 👀

[domnantas]

Sure, I can do that