// set logo
if (!has_custom_logo()) {
    // set logo of site to an image located at http://www.example.com/myimage.png
}

if( ! has_custom_logo() ) {
  add_filter( 'wp_get_attachment_image_src', '__return_dummy_logo_src' );
  the_custom_logo();
  remove_filter( 'wp_get_attachment_image_src', '__return_dummy_logo_src' );
}

function __return_dummy_logo_src( $src ) {
  return 'https://example.com/image.jpg';
}