Another attempt to fix crash in debug mode
Commit 97c03e0f was reverted 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