fix(fela-plugin-extend): don't throw on undefined items in extend array

[TxHawks]

Description

Fixes #657 where fela-plugin-extend would throw when the extend key in a style object was an array with a null or undefined item inside it.

Adds a test

Packages

fela-plugin-extend

Versioning

• Major
• Minor
• Patch

Checklist

Quality Assurance

You can also run yarn run check to run all 4 commands at once.

• The code was formatted using Prettier (yarn run format)
• The code has no linting errors (yarn run lint)
• All tests are passing (yarn run test)
• There are no flow-type errors (yarn run flow)

Changes

If one of the following checks doesn't make sense due to the type of PR, just check them.

• Tests have been added/updated
• Documentation has been added/updated
• My changes have proper flow-types

[robinweser]

LGTM! Feel free to merge and release. All you need to do is npm run release and select "patch" and it will update all packages and push them.

[TxHawks]

Can't release because flow errors out on unrelated issues (all of them in _book)

[robinweser]

Hmmmm, experienced the same issue with Flow in another repo and I couldn't even ignore them. Feel free to disable flow for the release.

Oh, and adding a quick note to the Changelog would be cool. Feel free to also merge #660 and add that one :)

[TxHawks]

updated the changelog, but I can't merge it without you approving it