Timestamp returns a different type depending on whether the data is empty or not.

#### Environment details

- OS type and version: macOS sonoma 14.6
- Python version: 3.11.9
- pip version: I use rye instead of pip. and rye version is 0.35
- `pandas-gbq` version: 0.26.1
- `pandas` version: 2.2.2

#### Code example

```python
import pandas_gbq

df = pandas_gbq.read_gbq('SELECT timestamp FROM `bigquery-public-data.pypi.file_downloads` LIMIT 1')
print(df.dtypes)

df = pandas_gbq.read_gbq('SELECT timestamp FROM `bigquery-public-data.pypi.file_downloads` LIMIT 0')
print(df.dtypes)
```

got the following. I expected either `us` or `ns`

```
timestamp datetime64[us, UTC]
dtype: object
timestamp datetime64[ns, UTC]
dtype: object
```

When I downgrade pandas version to 1.5.3, the above both code returns `datetime64[ns, UTC]`.

Thanks!