Metadata is overwritten on thread update if not explicitly specified

[asvishnyakov]

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Describe the bug

If metadata is None (which is default value) on update_thread, it will overwrite saved metadata with empty object

https://github.com/aitsc/chainlit/blob/31fe84a212f5a68fd0816f1e259d012c6cfa99e7/backend/chainlit/data/chainlit_data_layer.py#L551