fix(core): remove unpivot requirement for non-empty row labels

Author: shuoweil

bigframes/core/blocks.py

@@ -3418,10 +3418,6 @@ def unpivot(
             array_value, type="cross"
         )
 
-    if not labels_array.column_ids:
-        # A valid unpivot operation requires at least one row label to disambiguate the output rows.
-        raise ValueError("unpivot requires non-empty row_labels")
-
     new_passthrough_cols = [column_mapping[col] for col in passthrough_columns]
     # Last column is offsets
     index_col_ids = [labels_mapping[col] for col in labels_array.column_ids[:-1]]