Thank you for sharing the screen shot and explaining!
You are 100% correct the tooltip is indicating that the null value is being changed by the component to an empty string of "" and this is not easy to work with and is not what you want to be sending to your database.
I have filed a feature request to our engineering team and this change to allow the date time field to be set to and remain null has been added to the roadmap for the Form component.
I will keep you updated on any further news I hear from the team 
For now the best work around might be to have a little bit of validation logic where if the value is an empty string, to not send the form data to the DB until you add in a moment object you create then or a null value you append in to the form data's object in the place of the empty string to then send in a query.