Access nested fields in HuggingFace datasets when a field appears to not exist
or always returns default values. Use when: (1) iterating a dataset shows
all values as False/None/empty for a field you expect to have data, (2) the
field exists in raw data but not in dataset objects, (3) working with
conversation datasets where metadata like 'features', 'emotion', 'metadata'
contain the actual data you need. Covers nested dict access patterns in
HuggingFace datasets library.