Don't permit schedule preview with zero study segments.  #3309.

Justification:

PSC does not support this (it will return a 400 response), so there's no
point in passing such requests through.
Don't ask for schedule data when no event templates exist.  Fixes #3309.

This commit also establishes a zero point for
Psc::ScheduledActivityReport: null (really, undefined) filter set and
zero activities.  I previously did not want to do this because I didn't
think there was a sensible zero for Psc::ScheduledActivityReport, but
_some_ zero is needed for this fix, and this definition doesn't seem to
be too bad.