Show parsing error when receiving empty form list

[seadowg]

Fixes this crash.

What has been done to verify that this works as intended?

New tests.

Why is this the best possible solution? Were any other approaches considered?

I did look at being able to detect that there is an empty Document rather than needing to catch a RuntimeException, but it didn't seem like there was a good way to do that with kxml2. It might be worth making a contribution there to change it so that we get a specific exception, but I'm also not sure if we should be using that library at all.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

There's not really much we can verify here without a broken server (I don't think there's a way to get empty 200 responses back from Central or Aggregate). Best thing to verify here is downloading and updating form - there shouldn't have been any changes in behaviour.

Before submitting this PR, please make sure you have:

• run ./gradlew checkAll and confirmed all checks still pass OR confirm CircleCI build passes and run ./gradlew connectedDebugAndroidTest locally.
• verified that any code or assets from external sources are properly credited in comments and/or in the about file.
• verified that any new UI elements use theme colors. UI Components Style guidelines