How to handle valueless attributes in shortcodes?

Use 0 and 1 as shortcode values and use that show or hide the HTML attribute.

Example with an element input and the attribute required:

function input_shortcode( $atts )
{
    $values = shortcode_atts(
        array (
            'type'     => 'text',
            'value'    => '',
            'name'     => '',
            'id'       => '',
            'required' => 0,
        ),
        $atts
    );

    $values = array_map( 'esc_attr', $values );

    return sprintf(
        '<input type="%1$s"%2$s%3$s%4$s%5$s>',
        $values[ 'type' ], // 1
        '' === $values[ 'name' ]     ? '' : ' ' . $values[ 'name' ], // 2
        '' === $values[ 'value' ]    ? '' : ' ' . $values[ 'value' ], // 3
        '' === $values[ 'id' ]       ? '' : ' ' . $values[ 'id' ], // 4
        0  === $values[ 'required' ] ? '' : ' required' // 5
    );
}

Please don’t use extract(). Ever.