sueblack

Ugh, these things happen, after writing for help you’ll find the solution by yourself.
Definately not the most elegant solution, but it did the trick for me. I modified the the_meta() to be like this:

function the_meta() {
	if ( $keys = get_post_custom_keys() ) {
		$smgv = 0;
		foreach ( (array) $keys as $key ) {
			$keyt = trim($key);
			if ( '_' == $keyt{0} )
				continue;
			if ( $smgv == 0 ) {
				echo "<ul class='post-meta'>\n";
				$smgv = 1;
			}
			$values = array_map('trim', get_post_custom_values($key));
			$value = implode($values,', ');
			echo apply_filters('the_meta_key', "<li><span class='post-meta-key'>$key:</span> $value</li>\n", $key, $value);
		}
		if ( $smgv == 1 ) {
			echo "</ul>\n";
		}
	}
}

My conclusion: despite the comments in post.php get_post_custom_keys() does NOT return NULL when there is no custom data.

I won’t delete this thread, maybe it will help someone in the future.