Addon-docs: Remove leading pipe if using raw value for Flow union

[phated]

Issue: N/A

What I did

I ran into this issue prior to preferring elements in Flow's union type. In Flow, a Union can be specified with a leading | character, which results in an extra empty string value once processed by storybook. This removes that extra character if we don't have elements (for some reason) and must use the raw value.

How to test

• Is this testable with Jest or Chromatic screenshots? Yep, added a couple jest tests.
• Does this need a new example in the kitchen sink apps? Nope
• Does this need an update to the documentation? Nope

If your answer is yes to any of these, please make sure to include it in your PR.

[shilman]

LGTM!