fix: apply typed NULL to unpivot for empty columns

Author: shuoweil

bigframes/core/blocks.py

@@ -3429,7 +3429,7 @@ def unpivot(
     for input_ids in unpivot_columns:
         col_expr: ex.Expression
         if not input_ids:
-            col_expr = ex.const(None)
+            col_expr = ex.const(None, dtype=bigframes.dtypes.INT_DTYPE)
         else:
             # row explode offset used to choose the input column
             # we use offset instead of label as labels are not necessarily unique

tests/system/small/test_multiindex.py

@@ -1492,19 +1492,6 @@ def test_multiindex_eq_const(scalars_df_index, scalars_pandas_df_index):
     )
 
 
-def test_count_empty_multiindex_columns(session):
-    df = pandas.DataFrame(
-        [], index=[1, 2], columns=pandas.MultiIndex.from_tuples([], names=["a", "b"])
-    )
-    bdf = session.read_pandas(df)
-
-    # count() operation unpivots columns, triggering the empty MultiIndex bug internally
-    count_df = bdf.count().to_pandas()
-    assert count_df.shape == (0,)
-    assert count_df.index.nlevels == 2
-    assert list(count_df.index.names) == ["a", "b"]
-
-
 def test_dataframe_melt_multiindex(session):
     # Tests that `melt` operations via count do not cause MultiIndex drops in Arrow
     df = pandas.DataFrame({"A": [1], "B": ["string"], "C": [3]})