Fix persisting null with ObservablePersistMMKV

[simontreny]

This PR fixes persisting null values in observable when using ObservablePersistMMKV.

Consider this piece of code:

const obs = observable<string | null>("foo");

syncObservable(obs, {
  persist: {
    name: "obs",
    plugin: ObservablePersistMMKV
  },
});

obs.set(null);

This logs the following error:

[Error: Second argument ('value') has to be of type bool, number or string!]

and the old value is restored on next launch.

This happens because the persist plugin tries to call storage.set(null) while it's not supported by react-native-mmkv

[simontreny]

On second thought, I think the root issue is how safeStringify() is implemented:

return value ? JSON.stringify(value, replacer) : value;

It means that if the value is false (false, 0, "" or null), it won't be serialized as a string. If this called JSON.stringify() directly, this would work properly as null would be serialised "null" which can be saved and restored with MMKV.

I'm not sure I understand the intent of safeStringify(), I would expect it to always return either a string or undefined.

[simontreny]

It probably also means that persisting 0 (or false) is broken too, as it will be persisted as a number but we will attempt to restore it with storage.getString()