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Is your feature request related to a problem? Please describe.
Setting nested repeated fields via the python-bigquery API works (despite not being documented by ArrayQueryParameter).
ArrayQueryParamter.to_api_repr has code to handle "RECORD" or "STRUCT" but doesn't check if the array is empty and so throw exceptions.

Describe the solution you'd like
I'd a way to define the schema for the array of records so that it doesn't matter that it's empty - I can still use the BQ python client. There appears to be solutions for typed languages googleapis/google-cloud-java#2678 as well as dynamic languages googleapis/google-cloud-ruby#4269 and losely typed languages googleapis/nodejs-bigquery#400.

Describe alternatives you've considered
Our current solution involves naming each individual field in the repeated struct (eg fieldA_0, fieldB_0, fieldA_1,...)
and sending over MANY query parameters. As there's a limit to the number of named parameters the API allows, this is far from perfect and makes our code significantly harder to read and understand.

Additional context

from google.cloud import bigquery
client = bigquery.Client()

def query(params):
    sql = "SELECT * FROM UNNEST(@params) as param"
    query_parameter = bigquery.ArrayQueryParameter(
                "params",
                "RECORD",
                [
                    bigquery.StructQueryParameter.positional(
                        bigquery.ScalarQueryParameter("key", "STRING", key),
                        bigquery.ScalarQueryParameter("value", "STRING", value),
                    )
                    for key, value in (params.items())
                ],
    )
    for row in client.query(sql, job_config=bigquery.QueryJobConfig(query_parameters=[query_parameter])):
        print(dict(row))

query({"one": "a", "two": "b"}) # works
query({}) # IndexError: list index out of range