ToggleGroupControl: Fix active background for zero value

[ntsekouras]

What?

Discovered while working on #66837.

ToggleGroupControl had a logic error that would affect the background of the active item, if its value was zero. Zero is a valid value for this component so this PR just fixes that.

Testing Instructions

1. Test a ToggleGroupControl with ToggleGroupControlOption having zero value
2. Observe that in this PR the active background color is set properly.
3. (Alternatively just check out my Discovered while working on DataViews: API for layout density support #66837 and remove this line of code that was added here.

[github-actions]

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Co-authored-by: ntsekouras <[email protected]>
Co-authored-by: mirka <[email protected]>

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[github-actions]

Flaky tests detected in 82336b3.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/11737501169
📝 Reported issues:

• [Flaky Test] Navigates the items list via UP/DOWN arrow keys #64334 in /test/e2e/specs/site-editor/dataviews-list-layout-keyboard.spec.js

[mirka]

Good catch, thank you!