It is impossible to store falsy values

It is impossible to store values such as `false` or `0` using this plugin.

Change the `||` here to `??`, or use `.hasOwnProperty` instead.
https://github.com/ropbla9/vue-reactive-storage/blob/4c515e451935c0a21743fb915199f5e02c1fef8a/src/make-storage.js#L7