ToggleGroupControl: Fix active background for empty string value

[Mamaduka]

What?

Closes #69925.
Similar to #66855.

PR fixes active ToggleGroupControlOption background when an empty string is provided, which seems to be a valid case, but was omitted from the check.

Testing Instructions

1. Run a test plugin via DevTools console.
2. Confirm that the "None" option is correctly selected.
3. Switching between options works as before.

Details

const {createElement: el, useState} = wp.element;
const {__experimentalToggleGroupControl: ToggleGroupControl, __experimentalToggleGroupControlOption: ToggleGroupControlOption} = wp.components;
const registerPlugin = wp.plugins.registerPlugin;
const PluginDocumentSettingPanel = wp.editor.PluginDocumentSettingPanel;

function TestToggleGroupControl() {
    const [value,setValue] = useState('');

    console.log(value);

    return el(PluginDocumentSettingPanel, {
        className: 'test-custom-select',
        title: 'Test Toggle Group Control',
        name: 'test-toggle-group-control'
    }, el(ToggleGroupControl, {
        label: "Test",
        value: value,
        isBlock: true,
        __nextHasNoMarginBottom: true,
        __next40pxDefaultSize: true,
        onChange: setValue
    }, el(ToggleGroupControlOption, {
        value: "",
        label: "None"
    }), el(ToggleGroupControlOption, {
        value: "next",
        label: "Next"
    })));
}

registerPlugin('test-toggle-group-control', {
    render: TestToggleGroupControl,
});

Testing Instructions for Keyboard

Same.

Screenshots or screencast

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Co-authored-by: Mamaduka <[email protected]>
Co-authored-by: t-hamano <[email protected]>
Co-authored-by: Sourav61 <[email protected]>
Co-authored-by: mirka <[email protected]>

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[t-hamano]

LGTM!

• Is it worth backporting this PR into 6.8.x release? See #69925 (comment)
• I'm not sure why the component changelog entry check is failing, but the change log entry should have been added correctly.

[Mamaduka]

Is it worth backporting this PR into 6.8.x release?

Yes. It's not a major refactoring.

I'm not sure why the component changelog entry check is failing, but the change log entry should have been added correctly.

Same. It could be a problem with the workflow. I'll keep an eye on it.

[t-hamano]

A backport PR for the wp/6.8 branch: #70000

[t-hamano]

Removing the Backport to WP Minor Release label as #70000 has just been merged into the wp/6.8 branch.