Improve DateTimePicker empty field behaviour (Follow-up from "Resolve "Switch to date time picker for mount & unmount actions"")

The following discussion from !120 (merged) should be addressed:

• @nils.brinckmann started a discussion: (+1 comment)

  I'm sorry, but I found another thing that doesn't seem to work as I expected:

  I can clear the field for the date (for the hierarchy tree) manually (I select the text and delete it), then open the dialog, click on apply. The value in the text field then stays empty.

  I expect it to be a little weird behaviour in the reactivity - as the date in the component ifself doesn't change at all.

Important

Also include work for #94 (closed)

Closes #94 (closed)