Fix NoMethodError when result_count == 0

Fix NoMethodError & return [] by DMM::Response::Result#items when return result_count == 0 response XML

example: 

````
<?xml version="1.0" encoding="euc-jp"?>
<response>
	<request>
		<parameters>
			<parameter name="api_id" value="f2mz4GS41HBFf9GBEpzR" />
			<parameter name="affiliate_id" value="hQuc2XkCNnzs-990" />
			<parameter name="operation" value="ItemList" />
			<parameter name="version" value="2.00" />
			<parameter name="timestamp" value="2013-11-29 02:12:28" />
			<parameter name="site" value="DMM.co.jp" />
			<parameter name="result_only" value="true" />
			<parameter name="keyword" value="ポエム" />
			<parameter name="service" value="digital" />
			<parameter name="floor" value="videoc" />
		</parameters>
	</request>
	<result>
		<result_count>0</result_count>
		<total_count>0</total_count>
		<first_position>0</first_position>
	</result>
</response>
````