Add test to `MerkleDbDataSource` where save is called with unexpected values

We found some cases where `saveRecords(...)` is called with negative values for paths and/or empty streams. We should write tests for the different potential inputs and validate the behavior of the DB. Ideally it should be resilient to such things. We don't know how many items are in the stream at the time we try to save.

Start at MerkleDbDataSource.saveRecords(...), then trace how negative path values and empty streams are handled before reaching the database. Add coverage for the different inputs described and validate the database behavior, including streams whose item count is not known in advance.

Written by the indexing model from the issue text.