Shortcode $content bahaving bad!

Hello guys, what’s up!

I am writing my first shortcode and although it’s working great basically, the $content part is behaving rather weird.

Whenever called without content and without attributes, like this:
[zr_id]
the shortcode should return the currently logged-in user id,
which however does not happen, since is_null($content) fails to return ‘true’…

Here is the code:

function zr_id_func( $atts, $content = null ) {
	extract(shortcode_atts(array('zkid' => null), $atts));

	if ( is_null($content) ) {
		if ( isset($zkid) )
			$user_ID = $zkid;
		else
			global $user_ID;
	}
	else {
		//return 'not null?';
		$user_ID = do_shortcode($content);
	}
	return get_field('zr_username', 'user_' . $user_ID);
}
add_shortcode( 'zr_id', 'zr_id_func' );

Any ideas? Even tried trimming $content like this:
if ( is_null($content) ) {...

Without any success – same result!

Your help will be much appreciated. Thanks!