ProgramSerializer should now return default image if there's no program page

[jkachel]

What are the relevant tickets?

Related to #1738

Description (What does it do?)

If the program doesn't have a page, the frontend will crash because it expects a featured image. This adds a default featured image if there's no page, so it doesn't.

How can this be tested?

1. Create a new program. (Manually using the Django Admin is the recommended method so you don't inadvertently get a page created or anything.)
2. Create a program enrollment for one of your accounts.
3. Log in and navigate to the dashboard.
4. Click the My Programs page. You should see the new program.

[collinpreston]

Nice