Fixes the problem that after calling DatePicker.setDate() changing the time has no effect

See https://www.woltlab.com/community/thread/308863-datepicker-setdate-nicht-vollständig/

Author: Cyperghost

ts/WoltLabSuite/Core/Date/Picker.ts

@@ -911,6 +911,7 @@ const DatePicker = {
     const data = _data.get(element) as DatePickerData;
 
     element.dataset.value = date.getTime().toString();
+    element.dataset.empty = "false";
 
     let format = "";
     let value: string;