the issue was that widget wrapper was using parent QDateTimeEdit::dateTimeChanged signal. It was connected both internally to QgsDateTimeEdit::changed and in the form to detect changes. When QgsAttributeForm was recreating the updated feature it was calling QgsDateTimeEdit::value() before changed() could update the value (i.e. setting mIsNull to false).
fix#17790
the widget now uses a special value text to display NULL rather than painting a QLineEdit on top
a press on the spinbox buttons (non-calendar mode) is catch to set to current date to avoid starting at the minimum date
Run clang-tidy modernize-use-override to remove all the redundant
virtual keywords from overridden methods, and add some missing
overrides.
Another benefit is that this has also added the overrides
on destructors, which will cause a build failure if a base
class is missing a virtual destructor.
