Build fails when CSS contains escaped space

### Link to the code that reproduces this issue https://codesandbox.io/p/sandbox/github/vercel/next.js/tree/canary/examples/reproduction-template ### To Reproduce 1. Add a style rule containing an escaped space character to the CSS, e.g.: ```css [data-attr=\ ] { color: red; } ```` 3. `pnpm build` ### Current vs. Expected behavior Expect build to pass but instead throws with: ``` HookWebpackError: Cannot read properties of undefined (reading '0') at makeWebpackError (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/webpack/bundle5.js:29:315788) at /project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/webpack/bundle5.js:29:106487 at eval (eval at create (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/webpack/bundle5.js:14:9224), <anonymous>:44:1) -- inner error -- TypeError: Cannot read properties of undefined (reading '0') at /project/workspace/static/css/b0041ae41d46d821.css:1:1 at Parser.attribute (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:84899) at Parser.parse (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:99055) at Parser.loop (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:98727) at new Parser (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:84494) at Processor._root (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/com…