Return empty source object for all entity audit events

[ktuite]

This was meant to be part of backend PR #1072 but i forgot to push it!!

Frontend PR getodk/central-frontend#932 expects a source for every entity audit event, even API events, where the source is currently just {}.

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

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

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?

Does this change require updates to the API documentation? If so, please update docs/api.md as part of this PR.

Before submitting this PR, please make sure you have:

• run make test-full and confirmed all checks still pass OR confirm CircleCI build passes
• verified that any code from external sources are properly credited in comments or that everything is internally sourced

[matthew-white]

It seems a little funny to have a source object that's completely empty, but I guess it'd also be funny not to have a source property at all. I bet it simplifies Frontend to be able to assume that there is always a source property.

[ktuite]

It seems a little funny to have a source object that's completely empty, but I guess it'd also be funny not to have a source property at all. I bet it simplifies Frontend to be able to assume that there is always a source property.

We will have to put something in there soon to distinguish between "API" bulk upload and API single operation when the source is not a submission.