12540 fix gutenberg type check

[moorscode]

Summary

This PR can be summarized in the following changelog entry:

• Fixes an issue where the metabox would not display on term edit pages when running the development build of Gutenberg.

Relevant technical choices:

• In the development build of Gutenberg wp.data is loaded on term edit pages but the core/editor selectors are not registered. This causes null to be returned which is then compared to undefined causing us to wrongly assume Gutenberg is fully available.

Test instructions

This PR can be tested by following these steps:

• Checkout Gutenberg master, build plugin and activate it.
• Go to create new Category.
• You should see the metabox and it should work as expected.

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

Fixes #12540

Replaces #12613

[IreneStr]

CR 👍

[IreneStr]

Acceptance 👎

#12590 is not fixed. I still have an empty metabox, and see the following errors:

[IreneStr]

Acceptance 👍

[IreneStr]

These commits have been cherrypicked to the new 11.0 releasebranch.