DatetimePicker: fix max time error when init value is empty (#8336)

1.no init data
2.click start time and comfirm
3.click end time and comfirm(the end time is not change)
4.error
reason:
not evaluate 'maxDate' value in step1
I will show more details in conversation

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

@@ -508,7 +508,7 @@
           this.minTimePickerVisible = visible;
         }
 
-        if (this.maxDate && this.maxDate.getTime() < this.minDate.getTime()) {
+        if (!this.maxDate || this.maxDate && this.maxDate.getTime() < this.minDate.getTime()) {
           this.maxDate = new Date(this.minDate);
         }
       },