When the Accept-Language is empty, it should return `null`.

Source

						public function testPreferredEmpty() {
		$available = array('fr', 'de');

		$request = new ActionRequest(array(
			'env' => array('HTTP_ACCEPT_LANGUAGE' => '')
		));
		$result = Locale::preferred($request, $available);
		$this->assertNull($result);
	}