perf: avoid extra API calls from to_dataframe if all rows are cached

Follow-up to previous PR, which cached the first page of
`getQueryResults`. If the first page is the only page (no `pageToken`),
then it is unnecessary to make extra API calls from `to_dataframe` or
`to_arrow` to the BigQuery Storage API.

@@ -1351,6 +1351,41 @@ def __init__(
 self._total_rows = total_rows
 self._first_page_response = first_page_response
 
+def _is_completely_cached(self):
+"""Check if all results are completely cached.
+
+This is useful to know, because we can avoid alternative download
+mechanisms.
+"""
+if self._first_page_response is None or self.next_page_token:
+return False
+
+return self._first_page_response.get(self._next_token) is None
+
+def _validate_bqstorage(self, bqstorage_client, create_bqstorage_client):
+"""Returns if the BigQuery Storage API can be used.
+
+Returns:
+bool
+True if the BigQuery Storage client can be used or created.
+"""
+using_bqstorage_api = bqstorage_client or create_bqstorage_client
+if not using_bqstorage_api:
+return False
+
+if self._is_completely_cached():
+return False
+
+if self.max_results is not None:
+warnings.warn(
+"Cannot use bqstorage_client if max_results is set, "
+"reverting to fetching data with the REST endpoint.",
+stacklevel=2,
+)
+return False
+
+return True
+
 def _get_next_page_response(self):
 """Requests the next page from the path provided.
 
@@ -1412,6 +1447,9 @@ def _get_progress_bar(self, progress_bar_type):
 def _to_page_iterable(
 self, bqstorage_download, tabledata_list_download, bqstorage_client=None
 ):
+if not self._validate_bqstorage(bqstorage_client, False):
+bqstorage_client = None
+
 if bqstorage_client is not None:
 for item in bqstorage_download():
 yield item
@@ -1503,14 +1541,7 @@ def to_arrow(
 if pyarrow is None:
 raise ValueError(_NO_PYARROW_ERROR)
 
-if (
-bqstorage_client or create_bqstorage_client
-) and self.max_results is not None:
-warnings.warn(
-"Cannot use bqstorage_client if max_results is set, "
-"reverting to fetching data with the REST endpoint.",
-stacklevel=2,
-)
+if not self._validate_bqstorage(bqstorage_client, create_bqstorage_client):
 create_bqstorage_client = False
 bqstorage_client = None
 
@@ -1687,14 +1718,7 @@ def to_dataframe(
 if dtypes is None:
 dtypes = {}
 
-if (
-bqstorage_client or create_bqstorage_client
-) and self.max_results is not None:
-warnings.warn(
-"Cannot use bqstorage_client if max_results is set, "
-"reverting to fetching data with the REST endpoint.",
-stacklevel=2,
-)
+if not self._validate_bqstorage(bqstorage_client, create_bqstorage_client):
 create_bqstorage_client = False
 bqstorage_client = None