QSpinBox: clearing the value and pressing Enter should mean 0, not revert

Especially when used for editing integers in model/view, QSpinBox should behave like users expect from spreadsheets: if you clear the text and press Enter, then you expect to see 0, not the old value re-appear.

Testcase:
examples/widgets/itemviews/spinboxdelegate/spinboxdelegate
double-click in the cell that says '6'
type Delete
type Enter
-> 6 reappears, unexpectedly.