Workaround null-reference exceptions from non-frontend calls

Added null-reference check for `UmbracoContext.PublishedContentRequest`.

This applies to when the model-factory is called from the back-office context (e.g. a Save & Publish call).
Fixes #14 :suspect:
Wondering whether to replace with a check for `UmbracoContext.IsFrontEndUmbracoRequest` instead? (or have both?)