Hi,

When attempting to pass None as a value to represent a NULL in a BigQuery stored procedure, the process fails with a NoneType object is not iterable error. The issue is observed while using google-cloud-bigquery==3.25.0.

Steps to Reproduce:

1. Initialize a BigQuery stored procedure that accepts NULL values in its arguments.
2. Use the following code to attempt passing a NULL value to the procedure:

arg_values = None  # Representing NULL
bigquery.ArrayQueryParameter("arg_name", "STRING", arg_values)

3. Execute the stored procedure from Python code.

Actual Behavior:

The procedure fails with the following error message:

'NoneType' object is not iterable
Error observed in the file: venv/Lib/site-packages/google/cloud/bigquery/query.py [line: 754]

Expected Behavior:

There should be a way to pass NULL to arguments. For example, None could be accepted as a valid input to represent NULL in the BigQuery procedure, allowing for proper execution without raising any exceptions.

Regards,