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.
issue