Allow returning empty results when nothing is retrieved from DatabricksRM

[chenmoneygithub]

Right now in the following code, it's possible that key data_array doesn't exist, which leads to a opaque error to users. This happens when Databricks vector search index is not ready. We should not fail when this happens, and just return empty results.

        for _, data_row in enumerate(results["result"]["data_array"]):
            item = {}
            for col_name, val in zip(col_names, data_row):
                item[col_name] = val
            items += [item]