test: update empty agg test style and imports

Author: shuoweil

tests/system/small/test_dataframe.py

@@ -19,6 +19,8 @@
 import typing
 from typing import Dict, List, Tuple
 
+from bigframes_vendored.ibis.backends.sql.compilers import bigquery as bq_compiler
+import bigframes_vendored.sqlglot as sg
 import geopandas as gpd  # type: ignore
 import numpy as np
 import pandas as pd
@@ -6298,18 +6300,13 @@ def test_agg_with_dict_containing_non_existing_col_raise_key_error(scalars_dfs):
         bf_df.agg(agg_funcs)
 
 
-def test_empty_agg_projection_succeeds():
-    # Tests that the compiler generates a SELECT 1 fallback for empty aggregations,
-    # protecting against BigQuery syntax errors when both groups and metrics are empty.
-    import importlib
+def test_empty_aggregation_generates_constant_fallback():
+    # Given a BigQuery compiler and a parent table
+    compiler = bq_compiler.BigQueryCompiler()
+    parent_table = sg.table("parent_table")
 
-    bq = importlib.import_module(
-        "third_party.bigframes_vendored.ibis.backends.sql.compilers.bigquery"
-    )
-    sg = importlib.import_module("third_party.bigframes_vendored.sqlglot")
+    # When visiting an Aggregate operation with empty groups and metrics
+    res = compiler.visit_Aggregate("op", parent=parent_table, groups=[], metrics=[])
 
-    compiler = bq.BigQueryCompiler()
-    res = compiler.visit_Aggregate(
-        "op", parent=sg.table("parent_table"), groups=[], metrics=[]
-    )
+    # Then it should generate a SELECT 1 fallback to avoid BigQuery syntax errors
     assert "SELECT 1" in res.sql()