object dtype when empty query result

[yokomotod]

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Environment details

• OS type and version:
• Python version: python --version: Python 3.13.2
• pip version: pip --version: pip 25.0
• google-cloud-bigquery version: pip show google-cloud-bigquery: 3.30.0

Steps to reproduce

>>> from google.cloud import bigquery

>>> bigquery.Client().query("SELECT * FROM (SELECT 1 as integer_field) WHERE 1 = 0").result().to_dataframe().dtypes
integer_field    object
dtype: object


>>> bigquery.Client().query("SELECT * FROM (SELECT 1 as integer_field) WHERE 1 = 1").result().to_dataframe().dtypes
integer_field    Int64
dtype: object

It uses subquery SELECT 1 as integer_field to skip table setup, but it's same on normal query from tables.

cause

https://github.com/googleapis/python-bigquery/blob/v3.30.0/google/cloud/bigquery/table.py#L2642