DatePicker: fix initial value of [] in range type

packages/date-picker/src/panel/date-range.vue

@@ -155,7 +155,7 @@
 
   const calcDefaultValue = defaultValue => {
     if (Array.isArray(defaultValue)) {
-      return new Date(defaultValue[0]);
+      return defaultValue[0] ? new Date(defaultValue[0]) : new Date();
     } else {
       return new Date(defaultValue);
     }