Draft: [kcm/lookandfeel] Properly reset kdedefaults values when new LnF doesn't have a config value
When switching LnF we must clear any old value from the previous LnF in kdedefaults. However we explicitly check for values being empty and don't apply them then. Furthermore we explicitly check whether the new LnF has these values are set and don't apply them then. Both of these must not be done