fix error serving with older vite versions when config.css was not defined

[mruthh]

Summary

When attempting to serve a Vue app using Vite 4.3.5 with the tailwind/vite plugin, I received the following error:

[vite] Internal server error: Cannot read properties of undefined (reading 'devSourcemap')
  Plugin: @tailwindcss/vite:generate:serve

The issue was a missing ?. check. My vite.config file did not contain a css property and that crashed the plugin.

Test plan

• In pnpm-workspace.yaml, change the vite version to ^4.3.5.
• Run pnpm install
• In playgrounds/vite, run pnpm build && pnpm vite
• The example app should serve normally. Note that without the change from the PR, it fails to serve.