This used to be possible in 3.0 but was disallowed back in 29bf859fae
(Check if new index is in range before selecting new value in
wxPGProperty::SetChoiceSelection., 2014-11-29). However there doesn't
seem to be any real harm in still allowing to do this, so handle -1 as a
special case here.
Closes#24279.
Closes#24281.