BlankConnection types are broken in web template

[ben-polinsky]

Bad user experience trying to use blank connections due to poor typing: location and extents are required, but not reflecting in Viewer props - this causes the gatherRequiredViewerProps hook to fire and throw.

In Routes.tsx we check that a imodel id is present on the env and throw if not. This is not required for blank connections and is confusing.

I think we should also add some documentation in the template readme on how to get started with a blank connection.