Check for an empty user login in the $_REQUEST
array:
if ( empty( $_REQUEST['user_login'] ) // && or || empty( $_REQUEST['user_pass'] )
return $redirect_to_whereever_you_want;
But this filter is to late. It is called after the user is logged in. You have to use the 'wp_authenticate'
action, this is one of the earliest action in the login process:
add_action( 'wp_authenticate', '_catch_empty_user', 1, 2 );
function _catch_empty_user( $username, $pwd ) {
if ( empty( $username ) ) {
wp_safe_redirect( $redirect_to_whereever_you_want );
exit();
}
}