Fix issue when extending webpack config

[fvgs]

Issue: #1467

What I did

Fix bug leading to property access on undefined value

How to test

Run storybook after extending the webpack config with the following:

webpack.config.js

const webpack = require('webpack');

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

(Assumes jquery is installed)

[codecov]

Codecov Report

Merging #1468 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1468   +/-   ##
=======================================
  Coverage   14.54%   14.54%           
=======================================
  Files         202      202           
  Lines        4649     4649           
  Branches      499      504    +5     
=======================================
  Hits          676      676           
+ Misses       3548     3535   -13     
- Partials      425      438   +13

Impacted Files	Coverage Δ
app/react/src/server/config.js	0% <ø> (ø)	⬆️
.../ui/src/modules/ui/components/left_panel/header.js	28.57% <0%> (ø)	⬆️
addons/info/src/components/Props.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/api/actions/api.js	49.42% <0%> (ø)	⬆️
app/react-native/src/bin/storybook-build.js	0% <0%> (ø)	⬆️
app/react/src/client/preview/element_check.js	41.17% <0%> (ø)	⬆️
app/react/src/demo/Welcome.js	0% <0%> (ø)	⬆️
app/react-native/src/preview/story_kind.js	0% <0%> (ø)	⬆️
lib/ui/src/libs/key_events.js	23.25% <0%> (ø)	⬆️
addons/knobs/src/KnobStore.js	6.81% <0%> (ø)	⬆️
... and 16 more

---

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[shilman]

@fvgs if customConfig.module is falsy, isn't it also going to break on L75 above?

[fvgs]

@shilman I don't believe so. customConfig.module should be either an object or undefined. In the case of an object, the application of the object spread operator is fine. In the case of undefined, it's also fine. See Null/Undefined Are Ignored.

Testing with npm link, I have not observed additional issues.