Issue

When using postcss v7.0.20 the webpack compilation fails with the following stack trace.

✖ ｢wdm｣: TypeError: Cannot read property 'value' of undefined
    at /Users/username/workspaces/terra/terra-core/9.css:220:3
    at new Quoted (/Users/username/workspaces/terra/terra-core/node_modules/postcss-values-parser/lib/nodes/Quoted.js:19:28)
    at cloneNode (/Users/username/workspaces/terra/terra-core/node_modules/postcss/lib/node.js:15:16)
    at Quoted.clone (/Users/username/workspaces/terra/terra-core/node_modules/postcss/lib/node.js:248:18)
    at Func.Node (/Users/username/workspaces/terra/terra-core/node_modules/postcss/lib/node.js:83:30)
    at new Container (/Users/username/workspaces/terra/terra-core/node_modules/postcss/lib/container.js:45:18)
    at new Container (/Users/username/workspaces/terra/terra-core/node_modules/postcss-values-parser/lib/nodes/Container.js:15:1)
    at new Func (/Users/username/workspaces/terra/terra-core/node_modules/postcss-values-parser/lib/nodes/Func.js:19:5)
    at asClonedNode (/Users/username/workspaces/terra/terra-core/node_modules/postcss-custom-properties/index.cjs.js:254:21)
    at array.map.node (/Users/username/workspaces/terra/terra-core/node_modules/postcss-custom-properties/index.cjs.js:250:60)
    at Array.map (<anonymous>)
    at asClonedArray (/Users/username/workspaces/terra/terra-core/node_modules/postcss-custom-properties/index.cjs.js:250:48)
    at asClonedArrayWithBeforeSpacing (/Users/username/workspaces/terra/terra-core/node_modules/postcss-custom-properties/index.cjs.js:240:23)
    at root.nodes.slice.forEach.child (/Users/username/workspaces/terra/terra-core/node_modules/postcss-custom-properties/index.cjs.js:213:44)
    at Array.forEach (<anonymous>)
    at transformValueAST (/Users/username/workspaces/terra/terra-core/node_modules/postcss-custom-properties/index.cjs.js:193:24)
    at root.walkDecls.decl (/Users/username/workspaces/terra/terra-core/node_modules/postcss-custom-properties/index.cjs.js:275:28)

Details

I have confirmed that this commit introduced this error by reverting to v7.0.19 and verifying no error was thrown and then again by upgrading to v7.0.20 and verifying the error is thrown. Additionally I manually updated the code within node_modules while using v7.0.20 to revert to the previous looping logic and verified no error was thrown.

for (let i of defaults[name]) this.append(i)

We're still looking into exactly what is causing the error, but I wanted to make sure the issue was logged incase a quick solution/fix from the perspective of the maintainers of this library could be made.

Steps to Reproduce

(This is a rather large project, I'll work on creating a smaller sample project)

1. git clone https://github.com/cerner/terra-core.git
2. cd terra-core
3. npm install
4. npm start
5. Observe the stack trace above

• Project
• Webpack Config