Dataset handling if any of the keys have missing fields

If a hugging face dataset contains two keys, for example an input and context, and if either of the keys contain missing values, then we need to handle that by appending the populated fields to the input and the rest of the empty fields go away, so that all of the contexts are handled properly.

Locate the Hugging Face dataset handling entry point and reproduce a case where the input or context key contains missing values. The work is done when populated fields are appended to the input, empty fields are omitted, and all contexts are handled properly.

Written by the indexing model from the issue text.