Handle case when zero results are returned via find/describe

This edge case can occur in a newly provisioned server where there have been no published channels as of yet. A publisher will make a request to check the current metadata, the backend will return zero results and raise a RuntimeError.

Since this helper function record_batches_to_stream is only used in the context of creating metadata streams, we know what the schema is without looking at the incoming record batches, so I've simplified this. In the process, this handles the zero batch case well.