fix: handle empty index in _pd_index_to_array_value

Explicitly handle empty pandas Indexes in _pd_index_to_array_value to ensure
ArrayValue is created with correct columns, preventing crashes in unpivot
operations (like melt/stack) on empty or filtered DataFrames.

Author: shuoweil

bigframes/core/blocks.py

@@ -3471,6 +3471,28 @@ def _pd_index_to_array_value(
     Create an ArrayValue from a list of label tuples.
     The last column will be row offsets.
     """
+    if index.empty:
+        id_gen = bigframes.core.identifiers.standard_id_strings()
+        col_ids = [next(id_gen) for _ in range(index.nlevels + 1)]
+
+        data_dict = {}
+        if isinstance(index, pd.MultiIndex):
+            dtypes = index.dtypes.values.tolist()
+        else:
+            dtypes = [index.dtype]
+
+        for col_id, dtype in zip(col_ids[:-1], dtypes):
+            try:
+                bf_dtype = bigframes.dtypes.bigframes_type(dtype)
+                pa_type = bigframes.dtypes.bigframes_dtype_to_arrow_dtype(bf_dtype)
+            except TypeError:
+                pa_type = pa.string()
+            data_dict[col_id] = pa.array([], type=pa_type)
+
+        data_dict[col_ids[-1]] = pa.array([], type=pa.int64())
+        table = pa.Table.from_pydict(data_dict)
+        return core.ArrayValue.from_pyarrow(table, session=session)
+
     rows = []
     labels_as_tuples = utils.index_as_tuples(index)
     for row_offset in range(len(index)):

tests/unit/core/test_blocks_unpivot.py

@@ -0,0 +1,54 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from unittest import mock
+
+import pandas as pd
+import pytest
+
+from bigframes.core import blocks
+
+
+@pytest.fixture
+def mock_session():
+    session = mock.MagicMock()
+    session.bqclient = None
+    return session
+
+
+def test_pd_index_to_array_value_with_empty_index_creates_columns(mock_session):
+    """
+    Tests that `_pd_index_to_array_value` correctly handles an empty pandas Index by creating
+    an ArrayValue with the expected columns (index column + offset column).
+    This prevents crashes in `unpivot` which expects these columns to exist.
+    """
+    empty_index = pd.Index([], name="test")
+
+    array_val = blocks._pd_index_to_array_value(mock_session, empty_index)
+
+    # Should be 2: one for index, one for offset
+    assert len(array_val.column_ids) == 2
+
+
+def test_pd_index_to_array_value_with_empty_multiindex_creates_columns(mock_session):
+    """
+    Tests that `_pd_index_to_array_value` correctly handles an empty pandas MultiIndex by creating
+    an ArrayValue with the expected columns (one for each level + offset column).
+    """
+    empty_index = pd.MultiIndex.from_arrays([[], []], names=["a", "b"])
+
+    array_val = blocks._pd_index_to_array_value(mock_session, empty_index)
+
+    # Should have 3 columns: a, b, offset
+    assert len(array_val.column_ids) == 3