fix some bug in cretePages

[buugaaga]

hello)
during creating conten in contentful i run into mistake in createPages.
gatsby-awesome-paginate accepts object. Key items of these object should be array, but if tag not have field post an error accurs. Thank)

[ryanwiemer]

Good catch. This will prevent the build from breaking which is a better developer experience.

Just an FYI that in this scenario where a tag is created but not linked with any posts yet a tag page will still be created. (see below)