Fix Filter decorator with nested key paths of null

[dbachrach]

There is a bug in the filter decorator when using nested key paths. If the nested key path value is null. The key path one level higher is written. As an example:

const simpleEngine = {
  save(state) {
    console.log('final state:', state);
  }
};

const basicFilteredEngine = storage.decorators.filter(simpleEngine, [
  ['a']
]);

const nestedFilteredEngine = storage.decorators.filter(simpleEngine, [
  ['a', 'first']
]);

const data = {
  a: {
    first: null,
    second: 2
  }
};

console.log('Simple Engine');
simpleEngine.save(data);

console.log('Basic Filtered Engine');
basicFilteredEngine.save(data);

console.log('Nested Filtered Engine');
nestedFilteredEngine.save(data);

This prints out:

Simple Engine
final state: {
    a: {
        first: null,
        second: 2
    }
}
Basic Filtered Engine
final state: {
    a: {
        first: null,
        second: 2
    }
}
Nested Filtered Engine
final state: {
    a: {
        first: {
            first: null,
            second: 2
        }
    }
}

As you can see, when the value of a.first is null, the saved data structure is corrupted.

This PR ensures that a null or undefined value will properly set the state tree.

[michaelcontento]

Fixed with working tests in b7bf6e5