BigQuery: LoadJobConfig.from_api_repr does not return config

[yan-hic]

https://github.com/GoogleCloudPlatform/google-cloud-python/blob/63b7245699510259f584bc3ca328a72af8c2120f/bigquery/google/cloud/bigquery/job.py#L677
misses return config. Hence cannot build based on a JSON schema file.