fix: topic label field default to indexed field if not supplied

[zanussbaum]

from changes: https://github.com/nomic-ai/nomic/blame/main/nomic/dataset.py#L1173
Defaulted to using topic_label_field but if NomicTopicOptions was created with no overrides, the topic_label_field is None. Set to indexed_field by default if not overridden and also handles case of image topic label field

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🚀	This description was created by Ellipsis for commit cb3b8d6

Summary:

Default topic_label_field to indexed_field if not provided and handle image topic label field case in AtlasDataset.create_index method.

Key points:

• Default topic_label_field to indexed_field if not provided in nomic/dataset.py AtlasDataset.create_index.
• Handle image topic label field case in nomic/dataset.py AtlasDataset.create_index.
• Update examples/map_text.py to reflect changes in AtlasDataset usage.
• Increment version to 3.0.39 in setup.py.

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