cloud/pubsub: Fix failing integration test for messages with nil data
cloud/pubsub: Fix failing integration test for messages with nil data

Pub/Sub service seems to ignore the message published with nil data.
Testing against the case whether we can pull a nil message is obsolete.

Change-Id: I4b4632a5cb90968cecdded909b1d5fee9089f852

googleapis/google-cloud-go,googleapis/google-cloud-go,Maksadbek/gcloud-golang,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,ptomasroos/gcloud-golang,roth1002/gcloud-golang,Maksadbek/gcloud-golang,blowmage/gcloud-golang,hsinhoyeh/gcloud-golang,ptomasroos/gcloud-golang,blowmage/gcloud-golang,hsinhoyeh/gcloud-golang,michael-grosshans/google-cloud-go,roth1002/gcloud-golang