fix: fix ray sink error when there are no data to write

[SaintBacchus]

Reproduce python code:

import ray
from lance.ray.sink import LanceDatasink

ray.init()

sink = LanceDatasink("./data.lance")
ray.data.range(10).filter((lambda row: row["id"] > 10)).map(lambda x: {"id": x["id"], "str": f"str-{x['id']}"}).write_datasink(sink)

When using the lance ray sink to write lance file, the empty sink which may be caused by filter operator in ray data will cause these exception.

  File "/opt/conda/lib/python3.11/site-packages/ray/data/dataset.py", line 3621, in write_datasink
    datasink.on_write_complete(write_results)
  File "/opt/conda/lib/python3.11/site-packages/lance/ray/sink.py", line 141, in on_write_complete
    op = lance.LanceOperation.Overwrite(schema, fragments)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<string>", line 5, in __init__
  File "/opt/conda/lib/python3.11/site-packages/lance/dataset.py", line 1962, in __post_init__
    raise TypeError(
TypeError: schema must be pyarrow.Schema, got <class 'NoneType'>

The on_write_complete function assigns the schema by fragments. If there is no fragments, the schema will be None

[wjones127]

Thanks for the PR!

Would you be willing to add a test for this?

[wjones127]

We have ray integration tests here: https://github.com/lancedb/lance/blob/main/python/python/tests/test_ray.py

[westonpace]

(agree that a test would be useful)

[SaintBacchus]

OK, I will add a test case later

[SaintBacchus]

The new commit will not trigger the CI unit tests, please help me trigger it manually

[westonpace]

Thanks for adding the tests! A few small suggestions.