Properly account for deleted or archived Reference/Links

[jim-spot]

If a Contentful content type contains a reference to one or many other entries, it is possible for the reference to be undefined in cases where the referenced entry is archived or deleted from Contentful.

Example:

Let's say I have a blog post which contains a reference to one or more categories, it might look something like this pseudodata:

MyBlogPost
   Categories:
      -> Category 1
      -> Category 2
      -> Category 3

Let's say I delete or archive Category 2, the resulting pseudodata returned from contetnful will look like this:

MyBlogPost
   Categories:
      -> Category 1
      -> undefined
      -> Category 3

Today the rendered typescript interface looks something like this:

interface IBlogPost
{
   categories: ICategory[];
}

But in order to account for the above scenario, it should look like this:

interface IBlogPost
{
   categories: (ICategory | undefined)[];
}

Similarly for singular references, it currently looks like this:

interface IBlogPost
{
   category: ICategory;
}

But in order to account for the above scenario, it should look like this:

interface IBlogPost
{
   category: ICategory | undefined; // category?: ICategory is more syntactically correct, but was easier to implement this way.
}

Addresses: #86