test: add verification for empty selection aggregate projections

Author: shuoweil

tests/system/small/test_dataframe.py

@@ -6283,3 +6283,11 @@ def test_agg_with_dict_containing_non_existing_col_raise_key_error(scalars_dfs):
 
     with pytest.raises(KeyError):
         bf_df.agg(agg_funcs)
+
+
+def test_dataframe_count_empty_selection_succeeds(session):
+    # Tests that aggregate ops on empty selections don't trigger invalid empty SELECT syntax
+    df = session.read_gbq("SELECT 1 AS int_col")
+    empty_df = df[[]]
+    count_series = empty_df.count().to_pandas()
+    assert len(count_series) == 0