by ef55574a and then the crash
happens again.

I have tested this change and it does not break PalleteDocker.

PS: KIS_ASSERT_RECOVER_RETURN_VALUE trigger an assert and krita
fails to start up. KisPaletteModel::index works if I use
KIS_SAFE_ASSERT_RECOVER_RETURN_VALUE and change
KoColorSet::getGroup to return null pointer when its name
parameter is an empty string, but KisSwatchGroup::rowCount
crashes, so I think it is simpler to return an default
QModelIndex as soon as possible in KisPaletteModel::index

CCBUG:414890