In v0.4.0+, values of type undefined work differently - update documentation

[camsjams]

Using the todo list samples from the docs, we have the route get handling for genrelist[{integers:indices}].titles[{integers:titleIndices}] from falcor-router-demo here:

 return recommendationService.
        getGenreList(this.userId).
        then(function(genrelist) {
            var pathValues = [];
            pathSet.indices.forEach(function (index) {
                var genre = genrelist[index];
                
                // If we determine that there is no genre at the index, we must
                // be specific and return that it is the genre that is not 
                // present and not the name of the genre.
                if (genre == null) {
                    pathValues.push({
                        path: ['genrelist', index],
                        value: genre
                    });
                } else {
                    pathSet.titleIndices.forEach(function(titleIndex) {
                        var titleID = genrelist[index].titles[titleIndex];

                        if (titleID == null) {
                            pathValues.push({ path: ["genrelist", index, "titles", titleIndex], value: titleID });
                        }
                        else {
                            pathValues.push({
                                path: ['genrelist', index, 'titles', titleIndex],
                                value: $ref(['titlesById', titleID])
                            });
                        }
                    });
                }
            });
            return pathValues;
        });

The issue arises from this loose equality and the eventual usage of titleID:

if (titleID == null) {
    pathValues.push({ path: ["genrelist", index, "titles", titleIndex], value: titleID });
}
// null == null is true
// undefined == null is true

Unfortunately, on falcor-router v0.4.0, it seems that setting value to undefined will prevent the ref lookup, and return back empty data.

Instead this needs to be null.

For those following along with the tutorials, please advise that you will need to coalesce to null, or some other way to ensure that undefined does not make its way into your values!

var titleID = genrelist[index].titles[titleIndex] || null;

The documentation/tutorial does not match implementation anymore, although I can see why this change was made as it makes the value types more JSON-compliant.