Fixed crash when calling .simulate on an emotion styled component

[leroydev]

This fixes #12 for me locally. Without this fix, I had this error 6 times in a pretty big private codebase. This fix doesn't result in any problems in this codebase, while it does have ~1500 enzyme unit test cases and ~250 styled components. It also makes heavy use of Material UI V5, which also uses emotion. I made a minimum reproducible example here: https://github.com/leroydev/enzyme-adapter-react-emotion-issue and also included a way to test this fix by inlining ReactEighteenAdapter.ts with the proposed change.

After cloning this repo and running npm i, the old and new behavior can be compared by running npm run original-test/npm run changed-test 🙂