Allow templates for TCC not to have previous site

- Remove the null check on "PART_PreviousContentPresentationSite" when
  applying the template
- When enqueuing a transition test whether a previous presentation site
  exists in the template, if not set the content directly without any
  transition.
Merge pull request #532 from reactiveui/transitioningcontentcontrol-templates

Allow templates for TCC not to have previous site