fix: Non-number slideScale causing mispositioning

[dizys]

Fix #173 / #177.

Using null in number typed slideScale would also make Vue runtime type checking fail.

But notice this fix gives special meaning to value 0, replacing the original null.

[dizys]

I've tried replacing null with NaN instead, but NaN would require special comparison isNaN in order for SelectList in Settings.vue to work propertly with the checkmark class. So 0 is my best attempt so far.