sourceMap always prepend a undefined

[DavidKk]

I found that sourceRoot does not necessarily exist, sometimes it equals undefined.

I look for some codes, and found that extract-loader, it uses extractExports function to transform, (Function toString()) sandbox.module.exports.

And the css-loader use cssWithMappingToString and the variables cssMapping.sourceRoot to concat string, but css-loader has been remove the sourceRoot before cssWithMappingToString, so that cssWithMappingToString can not found sourceRoot anymore, and sourceRoot always to undefined

I suggest css-loader can compatibles it, when the sourceRoot is undefined, or keeps the sourceRoot option.

• cssWithMappingToString: https://github.com/webpack-contrib/css-loader/blob/master/src/runtime/api.js#L61
• css-loader: https://github.com/webpack-contrib/css-loader/blob/master/src/index.js#L76

Environment

• Operating System: osx 10.14.6
• Node Version: 12.6.0
• NPM Version: 6.11.3
• webpack Version: 4.41.4
• css-loader Version: 3.4.0

Expected Behavior

Would not prepend undefined

Actual Behavior

body{color:red}body a{color:#fff}body.open{background-color:#fef}
/*# sourceURL=undefined/src/index.scss */
...

Code

// package.json
{
    "extract-loader": "3.1.0",
    "css-loader": "3.4.0",
    "sass-loader": "8.0.0",
    "node-sass": "4.13.0",
}

// webpack config
{
  loader: 'extract-loader'
},
{
  loader: 'css-loader',
  options: {
    sourceMap: true
  }
},
{
  loader: 'sass-loader',
  options: {
    sourceMap: true,
    sassOptions: {
      sourceMapRoot: '/'
    }
  }
}