pubsub: cannot clear bigquery subscription

[hongalex]

In the same vein as #7979, there is a flaw with the logic of clearing BigQuery subscriptions. Setting a zero valued BigQuery subscription is a signal to clear the bigquery subscription, but right now Update passes on the empty BigQueryConfig instead of a nil one along with the update mask.

The flaw comes from

google-cloud-go/pubsub/subscription.go

Lines 849 to 850 in 372dd22

	if cfg.BigQueryConfig != nil {
	psub.BigqueryConfig = cfg.BigQueryConfig.toProto()

If BigQueryConfig is not nil (in this case, the sentinel clearing value), we immediately try to convert into the proto struct as is, rather than transforming it as nil. The fix here is to update the toProto method to convert a zero valued config to nil. To simplify, we only really need to check if config.Table is equal to the empty string, since we don't allow partial updates.

I'm not sure if PushConfig suffers the same problem or not, but will investigate this separately.

[hongalex]

Re: PushConfig, we don't have the same problem for some reason. Setting an empty PushConfig is enough to clear it though.