Hide custom fields when empty

There are a few issues that break your code:

• $post doesn’t exist, you’d need to import it using global, or simply use get_the_ID as you’re already doing
• $check[''] won’t return anything since it will access an undefined index, outside of the empty construct it would trigger a warning

I can’t test it right now, but this should work, at least syntax wise (and according to the API documentation theoretically also functionality wise):

function headway_title_button()
{
    $post_id = get_the_ID();

    if(metadata_exists('post', $post_id, 'titel_knop'))
    {
        printf(
            '<div class="title-btn"><a href="https://wordpress.stackexchange.com/questions/67540/%s" alt="https://wordpress.stackexchange.com/questions/67540/%s"><img src="https://wordpress.stackexchange.com/questions/67540/%s" /></a></div>',
            get_post_meta($post_id, 'titel_knop_url', true)
            get_the_title(),
            get_post_meta($post_id, 'titel_knop', true)
        );  
    }
}

However it might be more efficient to fetch all metadata at once. But WordPress isn’t smart enough to unserialize it when not requesting a single value, so you would need to do that on your own (if neccessary at all, depends on the metadata):

function headway_title_button()
{
    $meta = get_post_meta(get_the_ID());

    if(!empty($meta['titel_knop'][0]))
    {
        printf(
            '<div class="title-btn"><a href="https://wordpress.stackexchange.com/questions/67540/%s" alt="https://wordpress.stackexchange.com/questions/67540/%s"><img src="https://wordpress.stackexchange.com/questions/67540/%s" /></a></div>',
            maybe_unserialize($meta['titel_knop_url'][0]),
            get_the_title(),
            maybe_unserialize($meta['titel_knop'][0])
        );  
    }
}