Allow nullification of date and datetime fields

[erpas]

Once a value is set for a date or datetime field, it cannot be cleared or set to NULL. It would be useful to be able to clear it, as currently users can only modify it to another date/time.

[CharlesGag]

Related to that, you can't nullify a value when you use the "value relation" widget. Also, it would be nice to be able to allow null value using the same widget (ticking the box in QGIS doesn't seem to translate to Mergin App).