Fix: Logic for Highlight/text-color format availability

[dougwollison]

What?

Corrects the logic for showing the text-color format, either if a palette is present or if custom colors are allowed, as expected.

Why?

Previous logic mistakenly resulted in an unusable text color UI when palette was empty and custom was false, but offered no text color UI at all if palette was empty and custom was set to true.

How?

Flipped test on allowCustomControl.

Testing Instructions

1. Open theme.json
2. Under color settings, palette to [], custom to false
3. Open a post or page, insert a text block
4. Open the format dropdown, Highlight should be absent

For the inverse:

1. Set color.custom to true
2. Open editor, select/insert text block
3. Open format dropdown, Highlight should be present, selecting will allow custom color selection

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Unlinked Accounts

The following contributors have not linked their GitHub and WordPress.org accounts: @dougwollison.

Contributors, please read how to link your accounts to ensure your work is properly credited in WordPress releases.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Unlinked contributors: dougwollison.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

Warning: Type of PR label mismatch

To merge this PR, it requires exactly 1 label indicating the type of PR. Other labels are optional and not being checked here.

• Type-related labels to choose from: [Type] Automated Testing, [Type] Breaking Change, [Type] Bug, [Type] Build Tooling, [Type] Code Quality, [Type] Copy, [Type] Developer Documentation, [Type] Enhancement, [Type] Experimental, [Type] Feature, [Type] New API, [Type] Task, [Type] Technical Prototype, [Type] Performance, [Type] Project Management, [Type] Regression, [Type] Security, [Type] WP Core Ticket, Backport from WordPress Core.
• Labels found: .

Read more about Type labels in Gutenberg. Don't worry if you don't have the required permissions to add labels; the PR reviewer should be able to help with the task.