Skip validation when previous state is empty due to reset

[jdpgrailsdev]

What

Fix an edge case related to resets and syncs where at least one stream doesn't produce any data. The specific scenario plays out something like this:

• New connection created
• Connection is reset before any state is ever saved
  • This saves the state as LEGACY with a value of {"state":{}}
• Sync is attempted with more than one configured stream
• State save fails due to the validation on migration from legacy to per-stream state because not all streams produced data

How

• Ignore state migration validation if previous state is LEGACY AND is empty. In this case, we don't care if we overwrite the existing state, as it has nothing in it and we don't risk losing state.

Recommended reading order

1. PersistStateActivityImpl.java

Tests

• Added unit test to verify expected behavior
• All other tests pass
• Project builds locally

[benmoriceau]

Why do we bypass strict stubbing? Do we need those mock?

[jdpgrailsdev]

@benmoriceau I am not sure why this test has to do this. It is already creating lenient mocks and I originally attempted to not do that, but the tests fail without marking them lenient. In particular, these mocks are necessary, as we want to test the case where the previous state returned by the call to the state API returns a legacy state with an empty state JSON object for the data.

[benmoriceau]

Sounds good, let's not address that in this PR

[jdpgrailsdev]

@benmoriceau I discovered a failing acceptance test which lead me to add an additional null check. There is a scenario where the ConnectionState object may be non-null, but the state object stored within it is null. The code has been refactored a bit to make it easier to read. Specifically, take a look at the isStateEmpty method in PersistStateActivityImpl. I also added tests to cover the three different null/empty state scenarios. Please re-review at your convenience.