【v2.9.0-beta.6】empty less file throw an error

[tyouzu1]

Describe the bug

this change
result = await nodeLess.render
Read the less file if it is an empty file，result.map will be undefined,JSON.parse(result.map) will throw an error

here vite/src/node/plugins/css.ts#L1299

$ tsc && vite build
vite v2.9.0-beta.6 building for production...
✓ 583 modules transformed.
[vite:css] Unexpected token u in JSON at position 0
file: /***/index.less
error during build:
SyntaxError: Unexpected token u in JSON at position 0
at JSON.parse ()
at less (/node_modules/vite/dist/node/chunks/dep-7a3b37e3.js:19517:22)
at async compileCSS /node_modules/vite/dist/node/chunks/dep-7a3b37e3.js:19051:34)
at async Object.transform (/node_modules/vite/dist/node/chunks/dep-7a3b37e3.js:18691:55)
at async transform (/node_modules/rollup/dist/shared/rollup.js:21994:16)
at async ModuleLoader.addModuleSource /node_modules/rollup/dist/shared/rollup.js:22220:30)

Reproduction

x

System Info

x

Used Package Manager

yarn

Logs

No response

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[tyouzu1]

a pr
#7412

[patak-dev]

Fixed by #7412