Tweak how displayName is read in the shallow printer

[iChenLei]

What: fix #2321

Why: not necessary, just a patch fix.

How: add displayName to support better jest-test UX

Checklist:

• Documentation
• Tests
• Code complete
• Changeset

@fffed report a issue abount enzyme-serializer returns undefined at issue 2321.

exports[`enzyme shallow styled with css 1`] = `
.emotion-0 {
  background-color: black;
}

<div>
  <undefined
    className="emotion-0"
  >
    Test button
  </undefined>
</div>
`;

Then I try to debug emotion-js source code, and found why the serializer return a undefined!

noticed that emotionType not always be a string or a normal react component, it can be a forward component, React.forwardRef has displayName but no name

How to fix it ?

- typeof emotionType === 'string' ? emotionType : emotionType.name
+ typeof emotionType === 'string'
+   ? emotionType
+   : emotionType.name || emotionType.displayName

Now the jest unit test snapshot be more hunman readable.

exports[`enzyme shallow styled with css prop 1`] = `
.emotion-0 {
  background-color: black;
}

<div>
  <Button
    className="emotion-0"
  >
    iChenLei
  </Button>
</div>
`;

Hope emotion maintainer can code review this PR, thanks ! /cc @mitchellhamilton @Andarist

[changeset-bot]

🦋 Changeset detected

Latest commit: b3bdf84

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@emotion/jest	Patch

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Sandbox	Source
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Emotion issue template	Issue #2321