fix[parseTime]: fixed when pass null

https://github.com/PanJiaChen/vue-element-admin/commit/5890499077589db1c04c7a72c4dacaa97a58f87c

src/utils/index.js

@@ -9,7 +9,7 @@
  * @returns {string | null}
  */
 export function parseTime(time, cFormat) {
-  if (arguments.length === 0) {
+  if (arguments.length === 0 || !time) {
     return null
   }
   const format = cFormat || '{y}-{m}-{d} {h}:{i}:{s}'

tests/unit/utils/parseTime.spec.js

@@ -28,4 +28,8 @@ describe('Utils:parseTime', () => {
   it('empty argument', () => {
     expect(parseTime()).toBeNull()
   })
+
+  it('null', () => {
+    expect(parseTime(null)).toBeNull()
+  })
 })