Error when opening course when content list is empty

```
E/flutter (11877): [ERROR:flutter/lib/ui/ui_dart_state.cc(166)] Unhandled Exception: PlatformException(error, Invalid Query. A non-empty array is required for 'in' filters., null)
```

Reproduce the error by opening a course whose content list is empty, then trace the query that uses the reported 'in' filter. Done means the course opens without the PlatformException when the content list has no entries.

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