bug: error when no episode is published

Describe the bug

When no episode is published, an error 500 is raised on public website.

Steps to reproduce

1. Create a podcast.
2. Go to public website.
3. Error 500 lies there.

Expected behavior

No error 500.

Actual behavior

Error 500.

Relevant logs and/or screenshots

Context

• Castopod: alpha-37
• OS: Any
• Browser: Any
• Web server: Any

Possible fixes

getDefaultQuery() may return null but this is not handled.