Handle NaN input data (#241)

Sometimes LangChain would insert a single `[np.nan]` as a placeholder if
the embedding function failed. This causes a problem for Lance format
because then the array can't be stored as a FixedSizedListArray.

Instead:
1. By default we remove rows with embedding lengths less than the
maximum length in the batch
2. If `strict=True` kwargs is set to True, then a `ValueError` is raised
if the embeddings aren't all the same length

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Co-authored-by: Chang She <chang@lancedb.com>

python/tests/test_query.py

@@ -15,7 +15,6 @@ import unittest.mock as mock
 
 import lance
 import numpy as np
-import pandas as pd
 import pandas.testing as tm
 import pyarrow as pa
 import pytest