How to load google font only if custom logo is not uploaded

There is a function for this purpose, called has_custom_logo(). You can check whether the website has a custom logo or not by having this conditional:

if ( ! has_custom_logo() ) {
    // Enqueue some google fonts
    wp_enqueue_style( 'google-fonts', 'https://fonts.googleapis.com/css?family=Roboto:400' );
}