Fix issue when extending webpack config

[benbakhar]

Issue: #3215

What I did

Fix bug when property lookup is performed on undefined object

How to test

Run storybook after extending the webpack config with the following (Assumes jquery is installed):

webpack.config.js:

const webpack = require('webpack');

module.exports = {
  plugins: [
    new webpack.ProvidePlugin({
      $: 'jquery',
    }),
  ],
};

[igor-dv]

LGTM. Though #3261 will fix it in common

[Hypnosphi]

Do angular and polymer apps need the same fix for 3.4.0?

[igor-dv]

Probably they do