Pull request #3620: 无jira任务，cache.getItem默认值返回null，不要undefined，同原来的结果

Merge in VISUAL/fineui from ~GUYI/fineui:master to master

* commit 'efcd83a00f4cf53d9a3a5ee3dee10baabbc28c63':
  无jira任务，cache.getItem默认值返回null，不要undefined，同原来的结果

packages/fineui/src/core/structure/cache.js

@@ -13,7 +13,7 @@ export const Cache = {
         return Cache._getKeyPrefix() + (key || "");
     },
     getItem: function (key, opt = {}) {
-        const { defaultValue, typeConversion } = opt;
+        const { defaultValue = null, typeConversion } = opt;
         const storageKey = Cache._generateKey(key);
         let value = localStorage.getItem(storageKey);