Customizer Controls with a `type` of `hidden` receive a margin

[dlh]

WP_Customize_Controls with a type of hidden receive a margin-bottom: 8px, creating a slight gap when they are next to visible controls.

The attached patch removes the bottom margin on .customize-control-hidden.

Example code and screenshots:

function demo_customize_hidden( $wp_customize ) {
        $wp_customize->add_section( 'demo_section', array(
                'title'    => 'Example Section',
                'priority' => 10,
        ) );

        $wp_customize->add_setting( 'demo_first_text_setting' );
        $wp_customize->add_setting( 'demo_second_text_setting' );
        $wp_customize->add_setting( 'demo_hidden_setting' );
        $wp_customize->add_setting( 'demo_third_text_setting' );

        $wp_customize->add_control( 'demo_first_text_control', array(
                'label'    => 'Demo First Text Setting',
                'section'  => 'demo_section',
                'settings' => 'demo_first_text_setting',
                'type'     => 'text',
        ) );
        $wp_customize->add_control( 'demo_second_text_control', array(
                'label'    => 'Demo Second Text Setting',
                'section'  => 'demo_section',
                'settings' => 'demo_second_text_setting',
                'type'     => 'text',
        ) );
        $wp_customize->add_control( 'demo_hidden_control', array(
                'section'  => 'demo_section',
                'settings' => 'demo_hidden_setting',
                'type'     => 'hidden',
        ) );
        $wp_customize->add_control( 'demo_third_text_control', array(
                'label'    => 'Demo Third Text Setting',
                'section'  => 'demo_section',
                'settings' => 'demo_third_text_setting',
                'type'     => 'text',
        ) );
}
add_action( 'customize_register', 'demo_customize_hidden' );

Before:

After:

[SergeyBiryukov]

Looks like screenshots require an Evernote account, but the patch makes sense.

[dlh]

I've uploaded the screenshots here separately. Sorry, I thought I'd made them public on Evernote, but apparently not.

[SergeyBiryukov]

In 31460:

Customizer: Remove margin for hidden controls.

props dlh.
fixes #31330.