fix(user_report_viewed): Stops errors where course ID is 0. (#527 - Thanks @garemoko)

src/transformer/events/mod_forum/user_report_viewed.php

src/transformer/utils/get_activity/user_report.php

...user_report_viewed/existing_report_viewed/statements.json

...eport_viewed/existing_report_viewed_all_courses/data.json

...port_viewed/existing_report_viewed_all_courses/event.json

...viewed/existing_report_viewed_all_courses/statements.json

...report_viewed/existing_report_viewed_all_courses/test.php