fix: handle case when expirationMs is None

[abdelmegahed]

Right now, unsetting ['timePartitioning']['expirationMs'] on BigQuery table is not supported and raises google.api_core.exceptions.BadRequest: 400 PATCH https://bigquery.googleapis.com/bigquery/v2/projects/abdelrahmanm-bd-dev/datasets/partition_expiry_test/tables/sample_data_partitioned?prettyPrint=false: Invalid value at 'table.time_partitioning.expiration_ms.value' (TYPE_INT64), "None"

This happens since the backend doesn't treat "None" (string) as a null. This PR addresses this issue from the client side.

Sample code that fails with the above exception:

from google.cloud import bigquery
client = bigquery.Client(project='abdelrahmanm-bd-dev')
table_ref = client.dataset('partition_expiry_test').table('sample_data_partitioned')
table = client.get_table(table_ref)
table.partition_expiration = None # ISSUE HERE!! This get cast to string and the backend returns a 400 as a result.
client.update_table(table, ['partition_expiration'])

[tswast]

Thanks @abdelmegahed

Could you write a unit test covering this case? I'd expect one similar to https://github.com/googleapis/python-bigquery/blob/main/tests/unit/test_table.py#L1178 test_to_api_repr_w_custom_field perhaps called test_to_api_repr_w_unsetting_expiration

[abdelmegahed]

@tswast test added. Thanks for the pointers :)