Allow clients to determine whether data is intentionally unavailable

For instance, bandcamp users will always see when viewing streams and when viewing comments. The same applies to the "subscriber count", which is by design not provided by Bandcamp.

The reason is that the client (the NewPipe app) cannot differentiate between data that has failed to extract and data that is generally not available for the service.

There's an infinite amount of possible solutions to this:

• return -1 if extraction fails, -2 if not available by design
• return null if extraction fails, -1 if intentional
• return -1 if extraction fails, null if intentional
• return Optional.of(-1) if extraction fails, return an empty Optional if intentional
• add extra method call that determines which fields are filled by the provider (similar to MediaCapabiliy)
• …

If data is not available intentionally, NewPipe should hide the affected views. This makes the services that don't provide the corresponding data fields look less broken in the app.

Please comment what you think would be the best solution to this.

Start by reviewing how extractor results represent streams, comments, and subscriber counts, and how the NewPipe app renders missing values. Compare the proposed failure and intentional-unavailability semantics, including the existing MediaCapability approach, then seek agreement on one API design before defining completion criteria.