When inserting rows where a field A is a repeated Record, if A.a is None, and A.b. is 1, the fields are collapsed. The value of A.b is inserted into A.a instead of leaving A.a as Null.

from google.cloud import bigquery as bq

# my_table schema
schema = [SchemaField("Key"), "INTERER",
          SchemaField(
            "A",
            "RECORD",
            mode="REPEATED",
            fields=[
                SchemaField("a", "STRING"),
                SchemaField("b", "STRING")]

client = bq.Client()
tbl_ref = client.get_table(client.dataset('test-dataset').table('my_table'))
row = [{"Key":1, "A":{"a":None, "b":"not-null"}}]
client.insert_rows(row)

Result is a record in bigquery where key=1, A.a=’not-null’