Statistics: VoD view count broken

Describe the bug Instead of the VoD view count, a loading indicator is shown.

The server replies with an error 500:

{
  "error": "sql: Scan error on column index 0, name \"SUM(stats.viewers)\": converting NULL to int is unsupported",
  "message": "can not getcourse num vod views",
  "status": 500
}

To Reproduce E.g. course 892: https://live.rbg.tum.de/admin/course/892/stats

TUM-Dev / gocast

Statistics: VoD view count broken #1215