fix: [SQL] fetch schema if the dataframe schema is not set vs if the dataframe is null or not

[kavilla]

Once opensearch-project/OpenSearch-Dashboards#6957:

Convert dataframe service to a dataframes service

We should always expect there is a dataframe in the request. But using if the schema is set or not on to retrieve the schema. This ensures we don't wipe out the dataframe every request.