fix: repaire hexo-config() return value (#170)

hexo-config(item) will return empty string '' if item is set as 0 in _config.yml due to (0 || '') is '' in /lib/renderer.js:line 16
And some themes need the value 0 or errors would occurr

lib/renderer.js

@@ -13,7 +13,10 @@ function getProperty(obj, name) {
   let result = obj[key];
   const len = split.length;
 
-  if (!len) return result || '';
+  if (!len) {
+    if(result === 0) return result;
+    return result || '';
+  }
   if (typeof result !== 'object') return '';
 
   for (let i = 0; i < len; i++) {