"source" block for MySQL schema change events should contain db and table names

The source fields "db" and "source" are not populated for the records emitted to the user-facing schema changes topic. While that info can be derived from other fields of the events, it should be provided for the sake of consistency and uniform consumeability of events.

While add it, non-present optional fields shouldn't be emitted as null but rather just omitted from the events at all.