null check in sql preventing null to int conversion error (#1215)

[karjo24]

Motivation and Context

fixes #1215. Currently, the server throws a "cannot convert null to int"-error on the stats page, when no student has watched any vod or stream of this course yet. This is because there is no entry in the database referring to this specific course.

Description

Null check in SQL, returns 0 instead of null.

Steps for Testing

Prerequisites:
default testing database

Steps:

1. Before applying changes, go to http://localhost:8081/admin/course/1/stats. See that the "Vod Views" and "Live Views" show a loading indicator and a server error is thrown as described in issue.
2. Apply change
3. Reload, note that 0 is shown instead of loading indicator and no error is thrown.

[github-actions]

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Logins

Kurs1	Kurs2	Kurs3	Kurs4
public	public	loggedin	enrolled
prof1	prof1	prof2	prof1 prof2
student1 student2 student3	student1 student2	student2 student3	student1 student2