Invalid conditional when using `wp core update` without `--version`

See this block:

```
wp_version_check();
$from_api = get_site_transient( 'update_core' );
if ( empty( $from_api->updates ) ) {
	$update = false;
} else {
	list( $update ) = $from_api->updates;
}
```

The update proceeds even when `$update` is false.

Discovered when I saw these triggered errors:

```
salty-wordpress ➜  wordpress-trunk.dev  wp core update
PHP Notice:  Undefined property: stdClass::$version in /home/vagrant/.wp-cli/php/commands/core.php on line 901
Starting update...
PHP Notice:  Undefined property: stdClass::$version in /home/vagrant/.wp-cli/php/WP_CLI/CoreUpgrader.php on line 39
Downloading update from http://wordpress.org/wordpress-4.0-new-bundled.zip..
```