update:增加empty fn

src/utils/strlen.js

@@ -105,3 +105,10 @@ export default {
     return regTest.test(v);
   }
 };
+
+/*
+ * 判断字符串是否为null，undefined，空
+ */
+export const isEmpty = function(v) {
+  return v === null || v === undefined || v === '';
+};