fix: fix utc plugin diff edge case (#1187)

Author: iamkun

test/display.test.js

@@ -215,6 +215,10 @@ describe('Difference', () => {
     expect(dayjs('2018-08-08').diff(dayjs('2018-09-08'), 'month')).toEqual(-1)
     expect(dayjs('2018-01-01').diff(dayjs('2018-01-01'), 'month')).toEqual(0)
   })
+
+  it('undefined edge case', () => {
+    expect(dayjs().diff(undefined, 'seconds')).toBeDefined()
+  })
 })
 
 it('Unix Timestamp (milliseconds)', () => {

test/plugin/utc.test.js

@@ -269,3 +269,7 @@ it('utc keepLocalTime', () => {
   expect(dd).toEqual(dm)
   expect(vd).toEqual(vm)
 })
+
+it('utc diff undefined edge case', () => {
+  expect(dayjs().diff(undefined, 'seconds')).toBeDefined()
+})