kirki: css_vars font-family bug.

Hello. I found one error. When we use css_vars with font-family property, the result is bad or I’m doing something wrong.

Code:

	Kirki::add_field( 'id', [
		'type'        => 'typography',
		'settings'    => 'setting_id',
		'label'       => 'text',
		'section'     => 'section_id',
		'default'     => [
			'variant'     => 'regular',
			'font-family' => 'Playfair Display',
		],
		'transport' => 'postMessage',
		'choices'   => [
			'fonts' => [
				'google' => [ 'popularity', 30 ],
			],
			'variant' => [ '300', '300italic', 'regular', 'italic', '600', '700' ],
		],
		'css_vars' => [
			[ '--heading-font-family', '$', 'font-family' ],
		],
	] );

Suppose I chose Ubuntu font in Customizer.

Expected result: <style id="kirki-css-vars">:root{--heading-font-family:Ubuntu;}</style>

Current result: <style id="kirki-css-vars">:root{--heading-font-family:Ubuntu, ;}</style>

It adds a comma to the end of the font name. The question is whether there should be something else after this comma and I am doing something wrong or there should not be a comma at all (I think there should not be).

Greetings.