hierarchical term selector for flat taxonomies not working properly - problem with utils/terms.js

[yellowmastodon]

Description

If following the example in gutenberg/packages/editor/src/components/post-taxonomies/README.md to use the hierarchical term selector with a non-hierarchical taxonomy, non-hierarchical taxonomy does not render terms at all.

Problem lies in gutenberg/packages/editor/src/utils/terms.js. termsByParent is somehow not an array, but an object with one 'undefined' key.

So the return statement:
return fillWithChildren( termsByParent[ '0' ] || [] );
in function buildTermsTree() always returns empty array.

If the return statement is replaced with:
return fillWithChildren( Object.values(termsByParent)[0] || [] );
the terms function properly again.

It would be also nice if HierarchicalTermsSelector automatically removed rendering of dropdown for parent term if using non-hierarchical taxonomy. Will post a comment to the issue, if I am able to propose a solution.

Step-by-step reproduction instructions

1. add code to theme as per instructions in gutenberg/packages/editor/src/components/post-taxonomies/README.md
2. open any post in gutenberg editor
3. observe that taxonomies with flat terms do not render any terms

Screenshots, screen recording, code snippet

No response

Environment info

Wordpress version: 6.6.2
Theme: custom theme in development with no other changes to the gutenberg editor so far

Please confirm that you have searched existing issues in the repo.

• Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

• Yes

[torounit]

I was able to reproduce the problem with the following code:

src/index.js

import { PostTaxonomiesHierarchicalTermSelector } from '@wordpress/editor';
import { addFilter } from '@wordpress/hooks';

function customizeTrackSelector( OriginalComponent ) {
	return function ( props ) {
		if ( props.slug === 'track' ) {
			return <PostTaxonomiesHierarchicalTermSelector { ...props } />;
		} else {
			return <OriginalComponent { ...props } />;
		}
	};
}

addFilter(
	'editor.PostTaxonomyType',
	'my-plugin/set-hierarchical-term-selector',
	customizeTrackSelector
);

<?php
/**
 * Plugin Name: Issue 65704
 */

 add_action( 'admin_enqueue_scripts', function() {
    $asset_file = include plugin_dir_path( __FILE__ ) . 'build/index.asset.php';
    wp_enqueue_script(
        'issue-65704',
        plugins_url( 'build/index.js', __FILE__ ),
        $asset_file['dependencies'],
        $asset_file['version']
    );
 });


 add_action( 'init', function () {
    register_taxonomy(
        'track',
        'post',
        array(
            'public' => true,
            'label' => 'Track',
            'hierarchical' => false,
            'show_in_rest' => true,
        )
    );
 });