Fix the conditional check when retrieving the thumbnail image in course_type

[yash-learner]

Describe the bug
Fix the conditional check when retrieving the thumbnail image in course_type. The thumbnail method checks for the presence of the cover image when getting a course, this causes the thumbnail, not to render in school courses index page even though the course has a thumbnail image and no cover image.

To Reproduce
Steps to reproduce the behavior:

1. Create a course with a thumbnail but no cover image and refresh the page.
2. Course card will not have a thumbnail image even though it has a thumbnail image but you can see the thumbnail image on student pages

Expected behavior
The course card should have the thumbnail image.