Allow downtime records to be saved after clearing the end time value

https://schneiderampla.atlassian.net/browse/CE-2248

[System Info]
Ampla Versions: v7.2 onwards
Operating System: N/A

[Raised by]
Customer Company: Alcoa of Australia Limited
Customer Contact: wei.zhu@aveva.com

[Problem Summary]
Customer reported that they cannot add/save Downtime record after initiating the End Time value and resetting it to blank again.
Please see the screenshots.
Screenshot 1, Click the Date Time picker to set the value of End time, you can see at this moment, the record get validated with information 'This record has unsaved changes' and if you clear the value of End Time, there is an validation error 'The value is not of expected type. Type should be a system.DateTime' right away, please see Screenshot 2.

When try to add this record with no End time value, then Ampla reports "Data could not be saved to the server" error. Please see Screenshot 3.

But when you add a record with no End time value without initializing manually, then it can be successful.

[Reproduced in-house (Sydney GSC)]
Yes

[Steps to Reproduce]
1. In a green project, try to add a Downtime record in PA manually.
2. Click Date time picker to assign current date time to End time.
3. Clear the value of End time again
.4. Try to add/save the record, it reports an error as described above.

[Possible Cause]
Bug

[What is the impact to Customer/s]
Customer cannot add downtime records after resetting End Time to Blank.

[Workaround]
N/A
[Checklist]
Project backup
Data Repository
Event logs
Performance logs
Runtime Diagnostic logs
Screen dumps