storybook: Controls incorrectly populating an undefined option

Describe the bug Since upgrading to v6.2.5 all optional component props using string literals are also outputting an “undefined” option. Whilst technically accurate, this is causing confusion with users, particularly where default prop values are set that would make an undefined value an impossible outcome.

Using TypeScript and React. We’ve upgraded directly from v6.1.2 so unclear exactly when the behaviour changed.

To Reproduce Steps to reproduce the behavior:

1. Create a component using a string literal prop type
2. View in Canvas and look in Controls panel
3. undefined value should be present

Expected behavior I’d expect undefined to be ignored even if it was technically present on the typing (due to being optional), as has historically been the case.

Screenshots Incorrect in v6.2.5:

Expected behaviour as of v6.1.2:

System

  System:
    OS: Windows 10 10.0.19042
    CPU: (8) x64 Intel(R) Core(TM) i7-8650U CPU @ 1.90GHz
  Binaries:
    Node: 12.18.2 - C:\Program Files\nodejs\node.EXE
    npm: 6.14.5 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Chrome: 89.0.4389.114
    Edge: Spartan (44.19041.423.0), Chromium (89.0.774.68)
  npmPackages:
    @storybook/addon-a11y: ^6.2.5 => 6.2.5
    @storybook/addon-actions: ^6.2.5 => 6.2.5
    @storybook/addon-essentials: ^6.2.5 => 6.2.5
    @storybook/addon-links: ^6.2.5 => 6.2.5
    @storybook/node-logger: ^6.2.5 => 6.2.5
    @storybook/preset-create-react-app: ^3.1.7 => 3.1.7
    @storybook/react: ^6.2.5 => 6.2.5
    @storybook/theming: ^6.2.5 => 6.2.5