typography field: letter-spacing missing in CSS output if its value is 0

[rahulv3a]

Issue description:

If the default value of letter-spacing in typography field is 0, it does not output letter-spacing in CSS output. This causes the element to inherit letter-spacing from the parent elements.

Version used:

Latest commit in develop branch, 556c23d.

Using theme_mods or options?

theme_mods

Code to reproduce the issue (config + field(s))

Kirki::add_config( 'my_config', array(
  'capability'  => 'edit_theme_options',
  'option_type' => 'theme_mod',
) );

Kirki::add_field( 'my_config', array(
  'settings' => 'my_typography',
  'section'  => 'my_section',
  'label'    => esc_html__( 'Typography', 'example' ),
  'type'     => 'typography',
  'default'  => array(
    'font-family'    => 'Open Sans',
    'variant'        => 'regular',
    'subset'         => array( 'latin-ext' ),
    'font-size'      => '11px',
    'letter-spacing' => '0',
    'text-transform' => 'uppercase',
  ),
  'output'   => array(
    array(
      'element'  => '.test a',
    ),
  ),
) );