Type consistency remove NULLs

This is a ultra trivial observation

In simple_gmap_theme() the default twig variable definitions defaults to NULL

Now that we have a simple_gmap.schema.yml they should shadow the defaults in

SimpleGMapFormatter::defaultSettings()

function simple_gmap_theme() {
  return [
    'simple_gmap_output' => [
      'variables' => [
        'include_map' => NULL,
        'include_static_map' => NULL,
        'include_link' => NULL,
        'include_text' => NULL,
        'address_text' => NULL,
        'width' => NULL,
        'height' => NULL,
        'static_scale' => NULL,
        'url_suffix' => NULL,
        'zoom' => NULL,
        'link_text' => NULL,
        'map_type' => NULL,
        'langcode' => NULL,
        'static_map_type' => NULL,
        'apikey' => NULL,
        'iframe_title' => '',
      ],